I haven’t yet read anybody else’s answers yet, but I will after posting.
There are many, many reasons why one person (in this case this particular fellow) isn’t interested in you romantically.
They don’t feel the same chemistry that you do, even if you have tons of things in common. Even when you are older, you will come to find that even people that have what seems like almost everything in common, don’t necessarily click. That’s why I hate online dating sites, because common interests alone do not equate with chemistry (that mystery element that no one has been able to produce on command).
They might have just come out of a relationship with someone else. If the other relationship was bad, they may still be grieving and/or be disgusted with the idea of dating you or anyone else. If the relationship was good, they may hold every other person (including you) up to an impossible standard that you will never be able to fulfill.
They may have never been in a relationship with anyone because they are shy/introverted/boring/only interested in video games/would rather sit around eating Doritos than going to a movie with a girl (because it’s easier).
They may be interested in someone else (but have no intention of telling you that, because it’s none of your business).
They may have been treated poorly once, or multiple times, by the opposite sex and they are holding a grudge against all females for the time being. Or if you are particularly unlucky, the bad girlfriend may have bore a resemblance to you (not your fault).
They are not at the same level of emotional/physical develpment/maturity that you are. Even though this guy is in high school, some fellows peak much later (think of the guys on The Big Bang Theory).
They may have inadvertently heard some incorrect (unpleasant) information about you (that is false) that they think is true. Gossip is rampant in high school, so you may not ever know if he heard something, or if he did, from whom he heard it. Unfortunately for young fellows (and young ladies), once they hear something unpleasant about someone, even if it isn’t true, it often sours their opinion. This is one truly sucky situation, because there’s no way that you can really know where it originated from and you can rarely change people’s opinions (especially in high school).
It is also possible that your energy level, or your level of attractiveness, seem way out of range for him. Most people (I think) prefer to be with partners that are more similar (by comparison) with regards to perceived attractiveness, intelligence and energy levels. Quiet bookworms don’t tend to gravitate towards gregarious/boistrous/adventurous types and vice versa.
He might be a total guy’s guy. Meaning that although he is probably is attracted to females, at this point in his young life, he’d rather spend time playing basketball or video games with his male friends, because sometimes, when fellows are young, they think that the idea of having a girlfriend is just too much trouble, and not worth the time, money and aggravation.
He may perceive himself to be either way too smart, or way too dumb for you. Either one of those would not make a good match for either one of you.
If you are blonde, he might have a penchant for brunettes. If you are brunette, he might prefer redheads. If you’re a redhead, he might prefer blondes. If you have short hair, he might prefer long hair, if you have long hair, he might prefer short hair.
You might remind him of his mother or his sister, and that would give him an ick factor.
Maybe he thinks you have a weird laugh (I once put up with a fellow who brayed like a donkey, until I couldn’t take it any more, especially when he did that in a crowded movie theater, although it was bad enough in private). Maybe he thinks you have the wrong sounding voice (which, unfortunately for all of us would grate on all of us, depending on how bad we thought it was, and it’s completely subjective, so don’t get all concerned that you sound weird).
Maybe he liked you just fine as a friend (one of the guys so to speak) but now that he knows you’ve been harboring romantic fantasies about him, he’s totally weirded out, because he never considered you to be anywhere except the “friend zone.” Nobody likes to be in an un-equal partnership, where one person has “feelings” and the other one doesn’t. It’s awkward.
Maybe he likes one of your friends, or cousins, or your sister. That would be really awkward.
Maybe he’s asexual, meaning that he has no particular attraction to you or anyone else, and he’s totally OK with that, and or he enjoys his own company more than anyone else’s. Think of Sheldon Cooper on The Big Bang Theory.
Maybe he is truly acedemically inclined (of his own accord, or by insistence of his parents) that he concentrate on his studies and NOT dating.
Maybe he’s dating someone already (and it’s a secret, like she goes to another school) or maybe he’s even juggling multiple girls. Mabye he’s a player and he’s just waiting for something better (which means different things to different people) to come along.
And my personal favorite, maybe he’s gay. Been there done that (multiple times, when I was younger).
So there are hundreds of reasons why someone might not be interested in someone else. Rarely do they have to do with any inherent horribleness in the other person. Mostly it’s just that there is no chemistry. No harm, no foul. You’ll probably never forget this fellow, but you still need to move forward and look for someone who’s a better match. But don’t excpect to find that better match in high school, or while you are still in high school. It’s just not a realistic expectation.