Why Can't I Hate You?

I couldn't ever bring myself to hate you as I'd like

—The Stone Roses, "I Am the Resurrection"

Alice is in love with Bob. Unfortunately, she finds out that Carol is also in love with Bob, leading her to view Carol as an enemy. The catch? Carol is so damn nice to Alice that Alice feels guilty about disliking her. If Carol happens to be Bob's girlfriend, things can get even worse.

This can also apply to situations where someone simply has a non-romantic reason for hating someone, but said someone is too nice/charming to dislike. Can result in a Go-Karting with Bowser-like scenario if the participants in the Love Triangle end up hanging out a lot.

Compare Friendly Enemy.

Examples of the romantic version:

El Goonish Shive has Justin feeling like this toward Sarah.
In Sgt Frog, Tamama sees Angol Mois as a rival for Keroro's love and attention, but is frequently overwhelmed by her niceness. At one point in the dub, he thinks to himself regarding Mois "Stop making it so hard for me to hate you!"
- Mois also once turned Kululu into a shambling wreck by simply staring at him for an extended period. "I can't be mean to those eyes!"
Mahou Sensei Negima has a more complex version, with Yue trying to help her best friend Nodoka get closer to Negi...and then falling for Negi herself. She feels like a huge jerk for a while, thinking that she's betrayed her friend.
While there is no love triangle involved, in Knights of the Old Republic, when it comes to light that you're Revan, a talk with Carth reveals that he "tried to hate you, but couldn't." It's especially good to hear if you're a female PC seeking a romance sub-plot with him.
- Of course, he does give this exact same speach to the canonically male Revan.
In Bleach, during the Arrancar: The Arrival arc Orihime Inoue realized she was in love with Ichigo Kurosaki and felt this way toward her friend Rukia Kuchiki, who was also close to Ichigo.
On Friends Rachel sort of felt this about Julie. Monica had to remind her that Julie has done nothing wrong.
- This trope popped up practically every time Ross or Rachel had a Romantic False Lead, except in rare occasions where they really were jerks.
This happens on Karin. Winner is in love with Karin, who dislikes him because he's a vampire slayer (though it helps that he's utterly incompetent). Meanwhile, Karin's best friend, Maki, has a crush on Winner. Maki even says once, through thinly-veiled hypotheticals, that she wishes she could hate Karin for attracting Winner's attention, but Karin's too good a friend for that.
Ben Stiller's character feels this way about Owen Wilson's character, his fiance's ex-boyfriend, in Meet the Parents.
Pretty much the whole set-up of Reba. Barbara Jean is Brock's new wife but treats Reba, the ex, as a close personal friend which is what they eventually become.
My Name Is Earl: Earl is and always has been on friendly terms with Darnell (aka "Crab Man"), the man who cuckolded him and stole his wife.
In the book Avalon High, main protagonist Elaine can't bring herself to hate Jennifer, even though she cheated on her boyfriend, who happens to be Elaine's crush and wanted the best for Will. Same with Will himself. He realized his best friend and girlfriend loved each other and couldn't blame either them. It helps that the characters are actually the reincarnations of King Arthur, Guinevere and Lancelot.
Obligatory Buffy example. In one episode, Riley returns to Sunnydale after his breakup with Buffy and reveals that he's married. Willow decides that since it would be petty for Buffy to hate his wife, she'll do it for her. In the end, neither girl can hate her, because she's so cool and they're so perfect together.
In series 2 of Ashes to Ashes Jackie turns up at CID claiming to be pregnant with Gene's baby and Alex can only look on in horror as he proposes to Jackie. However Alex and Jackie bond over the episode and besides, why on earth would Alex affected by it? I mean it's not like she's got any feelings whatsoever for this glorified imaginary construct. It all gets better when Jackie reveals she lied to get Gene to help her and was insulted he believed she would sleep with him.
In Forgetting Sarah Marshall, Peter tells Aldous that while he has many reasons to hate him, the main one being that he's the guy Sarah dumped him to be with, he can't deny that Aldous is a cool guy too. But then it's subverted a minute later when Aldous lets slip that he and Sarah had begun seeing each other a year before she and Peter actually broke up, enraging Peter.
Spellman Files has this go on with Maggie. Isabel (and her sister Rae to some extent) don't really want her to settle down with Henry, but both Spellmans can't help but like Maggie. Luckily for them, Maggie breaks up with Henry and becomes their future sister-in-law instead.
The Taming of the Shrew is entirely about this trope. By extension, Kiss Me Kate and 10 Things I Hate About You.
In A Moment of Peace, Ito feels this way toward Crux on first meeting. Crux is the unrequited crush of his unrequited crush.
Discussed in The Stormlight Archive, when Navani tells Dalinar that she really tried to hate his (now deceased) wife, but couldn't because she was too nice.
On Jimmy Two-Shoes, Heloise begins dating Peep to make Jimmy jealous. It works, but to her suprise Jimmy and Peep hit it off quite well on non-Heloise topics.
On Sister, Sister, Lisa meets the woman who is apparently keeping Ray from getting together with her, and admits that she's conflicted about whether or not she should be angry at the woman or invite her into the house for a snack. It probably helps that the woman pulls a I Want My Beloved to Be Happy. (Made weirder because "the other woman" turned out to be Ray's dead wife, whom he felt reluctant to move on from.
In When Harry Met Sally..., this is Sally's response to Harry's Love Confession at the end.
Plan B takes this trope to its logical extreme with Bruno's attempts to sabotage Pablo's relationship with his ex Laura going awry when he finds himself liking Pablo as not only a best friend, but a love interest too.
In The Hunger Games trilogy, Gale admits to Katniss that he has a hard time hating Peeta because Peeta's so likeable, despite being a competitor for Katniss's affections.

Non-romantic examples

Also from Friends, Monica and the woman who stole her identity. Monica became friends with her.
A more disturbing example: Monster: "No matter what he did, I think I will forgive him". Yeah, Johan is just that charming.
Played with in interesting (and non-romantic) ways in The Things They Carried. Tim, a soldier in the Vietnam War, is best friends with The Medic, Rat. Rat gets shot, and transferred out of The Squad. Soon after, Tim himself is shot, and nearly loses his leg because Rat's replacement forgot to treat him for shock. Tim is rather bitter about this. Tim learns that the new medic is actually a nice guy who was simply inexperienced and cracked under the pressure of combat. Tim comments that if he could only bring himself to hate the new medic, he could also forgive him.
Both Physis and Jomy have this reaction to Keith Anyan in the last two episodes of Toward the Terra, each separately commenting that even though Keith has been doing everything he can to exterminate their race, they can't bring themselves to hate him. In both cases the most probable answer to Why Can't I Hate You? seems to be "Because he already hates himself enough for both."
Real life example: American tennis player Andy Roddick on Roger Federer, the man who's denied him multiple slams, most painfully one at Wimbledon 2009: "I've said to him before, 'I'd love to hate you, but you're really nice.'"
Bring Me the Head ofPrince Charming mixes this with an aversion of Evil Is One Big Happy Family—sloth, wrath, and pettiness are so rampant in the demonic community that the demon protagonist discovers he likes his angelic counterpart more than he likes most of his fellow demons. Until said angel hooks up with his ex-girlfriend and converts her to the side of light, that is . . .
In Squid Row, Randi is surprised to find her non-romantic male friend's new girlfriend is really nice and she likes her.
In Penny and Aggie, Duane, noticing for the first time the emotional vulnerability of The Fundamentalist Charlotte (who had attempted to frame a fellow student for making a bomb threat, and had slandered a lesbian student as a rapist), thinks to himself, "What is wrong with me, that I can't hate you now?"
In Atlas Shrugged, Hank Rearden is completely prepared to hate Francisco D'Anconia for his perceived worthlessness. When they actually have a conversation, Rearden realizes that they share a LOT in common.
In Tiger and Bunny, all the superheroes are in competition with each other, so it would be normal for them to resent the current King of Heroes, Sky High. If only he weren't so polite, earnest, and enthusiastic all the time.
In Airborn, cabin boy Matt loses out on a promotion to newcomer Bruce, who is much less experienced but whose father owns their airship (and plenty of others). After actually meeting him, Matt admits to himself that it would have been easier if Bruce was a spoiled brat - finding out that he's actually a decent guy just makes it worse.