Groundhog Day/WMG

With each loop/new day, an alternate universe is created.
In the time loop, when a day ends, the day does not disappear into nothingness. It becomes an alternate universe in which Phil exits the time loop through that day. Like the ending of the movie, when Phil exited the time loop through the day in which he is nice to everyone and lives with the consequences and results of that day, each day in the time loop has its own alternate universe in which Phil lives the results and consequences of that day. This means that there is an alternate universe where Phil promised to marry a woman he did not know, several alternate universes in which Phil has a bad relationship with Rita, several alternate universes in which Phil is dead because of suicide, and so on. None of the Phils know this though, and each Phil except for the first and true Phil has exited the time loop, and only the first and true Phil retains the knowledge he has gained from each day. Furthermore...

The first Phil never exited the time loop.
The ending of the movie only showed Phil exiting the time loop in one of the many alternate universes. In reality, the first and true Phil woke up the next day as he woke up almost every other day: In the same time loop. The time loop will always continue, and the first and true Phil will never escape and will forever create alternate universes in which a different, alternate Phil must deal with the consequences of their corresponding day.
 * Or not, considering how many days he killed himself for kicks.

The whole film is an inadvertant metaphor for Buddhism.
Harold Ramis admits he has a bit of a Buddhist bent, but it really hits the bullseye - getting "reincarnated" for 10 000 years? Only finding inner peace through compassion? Suicide won't kill you - you'll just get reincarnated? It's Buddhism to a T! Well... non-voluntary Buddhism, for Phil.

The snowstorm is part of the time loop.
It was originally supposed to hit Altoona and miss Pittsburgh entirely -- but when something decided that Phil Connors needed teaching a lesson, the weather was altered in order to strand him there. After all, so much more of a punishment to trap him in a small town he hates with limited options than trap him in a large city with plenty of alternatives to amuse / distract himself.

Ned Ryerson is a Time Lord.
After seeing for the first time what a huge douche his 'friend' was, Ned decided to teach Phil a lesson in humility. Much like the WMG below states, the loop was not broken by Phil's change of heart, but by his actual and sincere kindness towards Ned. May overlap with a very vague, temporal, Stalker with a Crush. "Ned: Phil, this is the best day of my life. Phil: Mine too. Rita: Mine too. Ned: Where are we going? Rita: Oh, let's not spoil it!
 * Another possibility is that Ned just wants to sell insurance. On the last day of the loop, Phil buys a major policy from Ned. Only after this purchase does Ned allow Phil out of the time loop.
 * Rita must be the doctor's companion!
 * That adds a lot of meaning to Ned saying "Whoa-ho-ho... that first step is a doozy!" on the first day.
 * OMG. That also adds meaning to Ned's last lines!

Ned: I got that. [noise]"


 * They're going to tomorrow!!!!!

The Infinite Time Loop had nothing to do with Phil Conners.
In another part of Punxsutawney, a Japanese transport college student by the name of Furude Rika ends up getting involved in some crazy shenanigans similar to an incident in her "childhood". The ending of the time loop had nothing to do with Phil Connors learning not to be a douchebag, but because she was able to prevent her own murder after several altenate time lines and countless Groundhog Days.
 * One problem with this though: Why was Phil Conners able to remember every further Groundhog Day? And why wasn't anybody else?
 * Because... Phil Conners is Furude Rika's Opposite Gender Clone! DUN DUN DUUUUUUUUUUUUUUUUUUUUUUUUUUN!
 * Because Phil Connors is an Integrated Data Interface, sent to earth to investigate the phenomenon that humans believe allows groundhogs to predict the future on a certain day of the year.
 * Because Phil Conners was sleeping in a room above the temporary Secret Storage Facility where the Stargate that cancelled the lack of Ripple-Effect-Proof Memory was being held.
 * Because Phil Connors is a ghost buster. Also, he was possessed by Cthulhu. Time ripples just do not compare.

The Phil Connors shown at the end of the film is not the real Phil Connors.
He got Bored with Insanity after slaughtering a few dozen (or hundred) townspeople a couple of times, and ended up either wandering through his days in a haze or being taken over by whatever was causing the Groundhog Day Loop.

Phil Connors is Old Spock from Star Trek (2009).
He's Old Spock from an alternate universe where he decided, unwisely, to try touching the Red Matter with his tongue. Groundhog Day is the unfortunate result. His escape at the end is a hallucination - he remains in Puxatawny forever.

Groundhog Day ends when Phil learns to put up with Ned Ryerson.
For some reason, karma, fate or whatever really wanted Phil to learn to put up with Ned. Even when he's improving himself, he's still does everything he can to avoid or drive away Ned, until the final iteration before the next day begins; after all, anyone can be nice to people they like or want to impress, but it takes character to act the same to someone you think is a complete douchebag.

Winning Rita's heart was just a happy bonus.
 * Couldn't be. Phil actually is nice to (and even hugs) Ned before his last loop. Ned actually runs away from him.
 * But Phil wasn't really being nice to Ned when he hugged him, he was trying to freak Ned out and get Ned to leave him alone that loop. So just hugging him wasn't enough to satisfy the universe if this hypothesis is true.

Phil is trapped in the world's most elaborate Choose Your Own Adventure plot ever.
Think about it. A number of events seem to happen in a specific order no matter how much time has passed or what you've done in the meantime. Phil can dilly-dally getting to Gobbler's Knob or he can jump out of bed and sprint there, and the same events will be happening. He can make minor choices, but the outcomes are few and simple. Slap, slap, slap, die, die, die.
 * This entry right here has made my day.

Groundhog Day is a video game
The player could end the loop at any time. He just wants to get the "Good ending", and keeps reloading the game at a save point (or possibly using save states). Like any player, he remembers his mistakes from last time and can try something different this time. Phil isn't in a timeloop, it's just the in-game explanation for why he can live the same day over and over and remember all previous iterations. The entire story is a massive case of Save Scumming.
 * But wouldn't he try focusing on the plot of the game instead of faffing about with stealing money and learning piano?
 * Yeah, why bother learning to race chocobos when you need to kill Sephiroth? (Final Fantasy)
 * This is actually not that hard to explain in gaming terms, either.
 * Don't forget...
 * This also explains the sudden burst of suicide. The player was probably frustrated by how hard the game was proving and engaged in a little therapeutic cruelty by ramming their character into every hazard they could think of.

The time loop was caused by a white hole.
After all, a white hole spews time back into the universe. After all, a white hole spews time back into the universe. Like just then, when time repeated itself.
 * So, what is it?
 * Only joking!

Rita is the cause of the time loop and she did not even notice.
I will give you a minute to laugh at it. Anyone find it weird that only after winning Rita's heart did Phil managed to break out of the loop? Plus, she shares the same name as a certain witch.

Phil did manage to return to Pittsburgh a few times, he just had to leave town the moment he woke up in the morning.
It just proved to be such an inconvenience he gave up at it. The blizzard would always hit while he was there and everywhere interesting closed.

The Powers That Be of the time loop prevented Phil from doing anything that would be too damaging to his psyche.
While daydreaming about all the cray things Phil could do with his powers is always an interesting thought experiment, he may have found himself unable to do things which would possibly haunt him forever - shooting up the town square, rape, cold-blooded murder, killing Rita herself - if the Powers That Be were intending for him to end up with Rita in the end they kept him from doing anything that would interfere with that end product of the loop.

The bartender was the cause of the infinite time loop.
The bartender, playing the role of the Magical Negro, was the person who caused the infinite time loop-- judging by the Knowing Glance towards the camera when Phil finally picks Rita's favorite drink and does her favorite toast.
 * I like the alternative idea that the bartender is also reliving Groundhog Day like Phil, but he just doesn't realize it. He probably gets the same customers every day anyhow, so he doesn't notice anything strange when it's the same routine with the same people--but he pays enough attention to them that he's happy to see Phil doing the right thing to impress Rita.

The time loop was started when Phil was hit in the head by the shovel after hanging up the phone.
This is the previous poster (the one about the bartender), and this is my sister's assertion.
 * Maybe Phil is experiencing all this in a coma brought on by the shovel hit!

Ned Ryerson is stuck in another (separate) time loop.
You know that scene where Phil randomly finds Nancy, asks her some basic information, then uses it to get her into bed the next time around? Ned did the exact same thing to Phil, implanting false memories in order to sell him insurance.

Phil was experiencing Haruhi's Endless Eight...
Only, he was feeling the delayed effects. This means that the last time was his Kyon moment.
 * Going along with that, he was created by the same organization that created Yuki.
 * Or, the Data Interface was just trying to gauge the reaction of a repeated period of time on a normal human.

Phil didn't go to sleep for years
It's pretty obvious he got a good night's sleep before the loop - he never seems to need to take a nap. So I'm assuming outside of times when he tired himself out, he didn't actually go to sleep very often.

Phil never got out

 * He just went insane, this time irrevocably.

Everyone has a vague subconscious awareness of the Loop, but only Phil was consciously aware of it.
At one point during the many attempts at setting up a perfect date, Rita comments to Phil that this seems somehow familiar and asks him about deja vu. It's possible that while nobody noticed enough for them to alter their set pattern for Groundhog Day barring intervention from Phil, there are phantom memories in the subconscious that could cause people Phil interacted with over and over to remember him on a level they can't access consciously. Over the period of the loop (whether you believe it is ten years or several thousand, it makes no difference) all the things Phil does right starts to build up, which is why all the people like Phil so much on the final day, even though he should be a stranger that performed a good deed for them earlier; subconsciously remembering years to centuries of Phil's kinder acts means people on the final day instinctively react to him as the town hero rather than someone who's only been there a day. This is also how Rita accepts Phil admitting he loves her this time compared to her previous reactions; it's not just detecting his sincerity this time, it's also remembering on some level the many repetitions where Phil started improving himself by being nicer to her.

Groundhog Day is part of a Source Code experiment.
The reason Phil gets stuck in a loop is because Colter is hijacking his mind to stop a terrorist attack. The mind has nowhere to go, so it goes back a day. Idea gets expanded on the link.

Groundhog Day takes place in the same universe as Replay
It's been eight years since Jeff Winston finished his Replays. Eventually Phil and Jeff's paths cross (they're both in the news industry) and they talk about their experiences.