Why is she so obsessed with this loser?

So no reason other than a pathetic self insert for masochist fags

Initially? Because she thought he was cute and he was fun to fuck with. Then all the time spent with him and interacting with him made her develop feelings. Not the most unbelievable way for a high school relationship to develop.

Because author needs to give self insert something good

Naga is a dork but also able to play the social game. She's probably actually interested in nerdy things. She also respects senpai for being able to follow his interests and stay true to his believes and character even when it means to be at the bottom of the social hierarchy. So she decides to get close to him and finds out that he is a hard working guy with decent views. In case you didn't know - that's quite the hook for the ladies and the reason why he's only a self insert at first glance. Most people hanging out here couldn't do what he does (and don't deserve a naga of their own). It helps that he is probably actually quite handsome.

She thinks she can fix him

Because slice of life/romance manga are shit and poorly written, and yet, you grown ass men keep reading this. I would be ashamed of myself if I were you.

Thank god your judgement doesn't matter

Cope, man-child.

huh? You seem butthurt

I don't get it either even tho I've been reading the manga since the start, they don't have anything in common and there is literally no reason for her to fall in love with him, It's just pure self insert.

Because she herself is a loser who gets mogged by every other girl she comes across.

All medium should be like this.
If the losers are the ones shaping the young females collective's subconscious, they should influence them into liking losers and old men, meanwhile painting good looking men as always narcisistic and frivolous.