I'm only intimately familiar with about half of the stories people are discussing, but just for my two cents, I have no problem with shipping, or seemingly mismatched pairings, but as far as the "Love at first sight" thing, it's a product of a simpler time that kind of needs to go away. Not to stir any sh*t, but it's been scientifically disproven multiple times. You can NOT fall in love at first sight. Love's a little bit more complex than, "OMG... Dat ass! I'm in LOVE!"

Attraction, sure, that definitely happens, but attraction isn't love.
In ye olden days - and especially in eras and places where things like teenage weddings and arranged marriages were common - it was a convenient idea to put in people's heads, that you can find your "soul mate" immediately. It made sense then, not only for society, but also in fiction. For starters, women "back in the day" weren't encouraged, or even allowed, to actually... you know... date around, meet new people, try and sort it all out for themselves, struggle with crippling doubts and insecurity... any of that stuff that happens in real life, y'know? For ages, both in fiction and real life, women were told, "Don't worry about any of that. When you meet The One, you'll just know. There'll be fireworks and rainbows shooting out of his ass, it's gonna be awesome. Love at first sight, it never fails."
Except, we're more sophisticated now than we were even 50 years ago, and we all know that's all BS. We've completely narrowed "love at first sight" down to, more or less, a primitive human urge to breed, and it fades incredibly fast. So, since we know it's not real, I'm less accepting of it in newer fiction than I am in stuff like, older Disney films, for example. I can handle how they told those stories back then, as a product of their time, and to be honest, they're charming. But we're past that as a species now. I *still* run into chicks - grown women - who swear they're gonna find a "Prince Charming" one day, and it's going to be just like a movie. I don't have any explanation for this, as it's completely asinine. And I can only assume it persists because some people still buy into these *highly fictional* explorations of "love" and relationships.
So, no, for me, explaining Donnie's crush - and its intensity, and how committed he is to it and how much of his time it occupies - by saying, "well, other great stories have worked the 'love at first sight' gimmick too, so, y'know, all's fair," doesn't really wash. It's an outdated concept that has no place in modern society, or storytelling, except to make the character who believes in it - Donnie, in this case - to look like a complete toolbox. It is kinda funny how someone so scientifically-minded as him could become so irrationally emotional over anything, but I'd argue his character isn't any *better* for it. It doesn't make him in any way endearing. I'unno. It just makes him seem like an irrational, needy, emotionally-desperate loony toon.