The setting is modern day Japan, and there are two main male protagonists. They were relatively close friends as children (though they bickered a lot), but circumstances irrelevant to this post separate them. They meet again several years later...and this is where things get tricky.
The older of the two, for purposes of the plot, becomes very devoted to his old friend's well being not long after the reunion, which goes badly as the younger one is, due to a major trauma in his life, not the person he once was; the past friendship doesn't mean anything to him anymore and as thus is quite hateful towards his former friend and his devotion.
So here's the problem: what is it about the younger guy, apart from their shared past, that would cause the older guy to continue to be so close to him despite being treated like dirt?
Here's what I DO know about the older guy (for greater context if it helps anyone):
1. He never knew his father. At all. His parents split before he was born. At this point I'm not sure whether or not his mom will remarry.
2. His childhood was rocky; he suffered from severe asthma and had few friends apart from this other guy. Things improved in both health and friendships as he matured, however.
3. He is heterosexual, though I could change that if need be.
Finally (and most importantly):
4. This devotion of his is not merely a desire to continue the friendship. He could probably care less. This is almost an obsession with the other guy, and (though I haven't decided yet) he may give up his life for him and has already come pretty close, though it still doesn't mean jack squat to the other (I like to think of the Bruno Mars song "Grenade" here). So what's up with him? Under what circumstances would a guy like this get so obsessed with a friend?
Research wise I have done little, though I did at one point search "why would a straight male become fixated on a male friend"...and I got a bunch gay porn sites. :P