Overwhelming Obsession

Dwarfs are very attached to gold. Any highwayman demanding 'Your money or your life' had better bring a folding chair and packed lunch and a book to read while the debate goes on.

—Men at Arms

Most people like particular things and dislike other particular things. Some people really like certain things, to the point that everybody else around them knows it. And then there are these people - the ones who really, really like something, to the point that the liking is a major facet of their personalities, verging on or actually being an Overwhelming Obsession with the thing that's liked.

On occasion, a Flat Character gets an Overwhelming Obsession as his only character trait. If this sort of character ever gets A Day in the Limelight, expect the character's obsession to be highlighted just as much at the same time. At worst, a flat character with an Overwhelming Obsession can exist simply to be used as the author's mouthpiece to show off his work... or his own hobbies. At best, their obsessions provide comedy relief, dramatic impetus to the story, or both. Well-rounded characters can also have an Overwhelming Obsession, though, fitting it into their personalities. These obsessions can be used to avoid an Ass Pull when a plot resolution requires the character to know or have something related to the obsession; the character knows or has whatever's needed because it relates to the character's obsession.

If this attraction is taken out of context - or, in some cases, in context - it can sometimes be seen by in-universe viewers as Head-Tiltingly Kinky.

Overlaps with Drink Order and Trademark Favorite Food (especially the Orgasmically Delicious sort), when those are what the character lives for. Compare with Security Blanket, where the liked item in question gives its owner peace of mind simply by being within sight. Not to be confused with Fetish, which is when someone really, really likes something in that very special way.

Examples of Overwhelming Obsession include:

Anime and Manga

Read or Die: Yomiko loves books, and the paper in them loves her back. The earliest manga chapters show her loving books romantically.
Mahoromatic: Chizuko really loves good cooking (but doesn't have a particular Trademark Favorite Food), to the point that she finds Mahoro's excellent cooking to be Orgasmically Delicious.
Mikoto in Is It Wrong to Try to Pick Up Girls in a Dungeon? loves hot springs so much that she can track them down by scent. For a while, this was her only defining character trait.
Ranma ½ has a few characters which exhibit this trait to one degree or another. Ukyo Kuonji is a mild case, having built her life around being an okonomiyaki cook and even creating a martial art using the implements of her craft. (As noted below in Fan Works, fanfic writers frequently turn this Up to Eleven when writing her.) And then there's Azusa Shiratori, a one-shot character from early in the story who is obsessed with all things cute; given half a chance, she will steal anything that catches her eye, give it a French name, and then insist from that point on that it is her property. About the only thing she cares about other than cute stuff is her ice skating.

Comic Books

Many of Batman's Rogues are crazies and weirdos who are obsessed with a concept or thing they base their entire identities around: Two-Face and duality, Scarecrow and fear, Poison Ivy and plants, Riddler and his... well, riddles... whether they're solely defined by these quirks or are deeper characters that happen to have an obsession differs from writer to writer, though D-Listers like Captain Blimp and Cap'n Fear are often stuck with being the former.
Marvel Comics's Thanos loves Death -- its Anthropomorphic Personification, not the act of killing, which for him is simply a means to the end of being able to see her again.

Fan Works

Ukyo Kuonji from Ranma ½ is frequently depicted as obsessed with being an okonomiyaki chef, to the point that she structures her fantasies about living a married life with Ranma around cooking okonomiyaki together in her restaurant.

Literature

Discworld has this as the dwarfs' racial attitude toward gold. In more than one novel, it's made clear that their sorrow after a mine cave-in isn't caused by the loss of dwarf life, but rather by the loss of a seam of gold that hadn't been completely mined out yet. As for keeping the gold once they've dug it out of the ground, see the page quote.
Hagrid from Harry Potter is a well-rounded character and important ally to Harry in his own right, but he's a little too obsessed with highly dangerous and aggressive magical creatures for his own good. Whether it's the ridiculously aggressive Blast-Ended Skrewts that he's bred into existence, the man-eating acromantula Aragog that he raised from infancy, or the various threatening creatures he keeps around for his Care of Magical Creatures classes, Hagrid conducts himself like an overly sappy, loving father to them all, which has gotten him in hot water more than once when his beloved creatures have hurt people or been framed for doing so.
In The Lord of the Rings, the major threat behind The One Ring is its ability to corrupt the minds of those who wear it, and in a few cases, see it. One only has to look at the friendly hobbit Smeagol's transformation into the utterly insane and twisted Gollum to see how deep its corruption runs. Gollum is consumed by his obsession with the Ring and lovingly refers to it as "my precious", a trait that Bilbo shares in the modern day which further convinces Gandalf that he's in danger of having his mind destroyed by it.

Live-Action TV

Carlton Lassiter from Psych really loves his guns. A lot. To the point that he keeps a ton of them stashed in various places in his house, including in a bowl of almonds. He's also very Trigger Happy and sees no problem in casually threatening to shoot people (namely his boss, Shawn) who annoy him, and when he's fingered as a murder suspect and has to give up his firearms, it contributes heavily to his resulting downward spiral.

Video Games

Like in the comics, Batman's rogues in the Arkham series of games are usually defined by their obsession with a concept or item. Ironically, while his love of birds takes a backseat to his sadism and greed, Penguin's love of birds and bird-themed collectibles leads to one random thug in Arkham City derisively referring to him as a "midget with a bird fetish".
To an outsider to Fire Emblem: Three Houses‍'‍s Golden Deer house, Raphael Kirsten's personality seems dominated by his love of two things: eating and training. And even if you choose to join the Golden Deer, a lot of Raphael's dialogue revolves around those two things. However, he's a surprisingly deep character in his own right as shown by his support conversations with others, it's just that he likes to focus on things that make him strong enough to protect the people he cares about, and it just so happens that the things that make him stronger are delicious food and rigorous workout routines.
While there are plenty of characters in the Monster Hunter series that love and appreciate the various monsters that populate the setting and want to learn more about them, the Guildmarm from the fourth game takes her obsession just a little too far. And by a little, it's more like she's literally in love with them. The explosive slime-wielding, brawling wyvern Brachydios is the main target of her affections, and when the Hunter is assigned quests to fight one, she waxes lyrical about how dreamy and attractive she finds them, and at one point admits to being the sole member of a Monsterphile Club. Needless to say, she may have a few screws loose here and there... but that's exactly why the fans love her.
- Hinoa from Monster Hunter Rise really loves to eat bunny dango. She brings it up a lot in casual conversations, is often snacking on the job, and when you meet the dango maker Yomogi, she sounds as if she's in love with them before getting a hold of herself.
Junkrat from Overwatch loves blowing things up. And by "love", we mean really, really loves to blow things up. His literal lust for destruction is so strong, that he acts like he's having an orgasm when he lights a fuse during his and Roadhog's attempt at robbing Junkertown in the short video titled "Junkertown: The Plan".

Western Animation

The SpongeBob SquarePants episode "To Love a Patty" has Spongebob literally fall in love with a Krabby Patty. He takes it on dates and romantic outings, oblivious to it rotting and the resulting ire he draws from people unfortunate enough to smell it.

Real Life

"If you love "X" so much, why don't you marry it?!" is a common bit of schoolyard teasing directed towards kids perceived as obsessed with something, usually a toy, favorite food, or other such items.
Most sexual fetishes tend to materialize with an erotic obsession with something way outside the norm, such as a specific body part or object as opposed to the male and female form as a whole. Several instances of rather extreme fetishes include a man being in a committed romantic relationship with his car, a 4chan anon developing a kink for tile patterns, and a man's obsession with women farting on cakes resulting in the creation of the infamous "cakefarts" video (link is to a documentary discussing the video's history). It goes without saying that each link is very NSFW.