Emotional instability, likely clinical depression, bipolar or a temporary chemical inbalance. It could be just a simple burnout (from being pressured by the lovely family into getting straight A's, living a perfectly happy life, and being perfect, etc). I really need more information to go on.
That someone has straight A's, or what you percieve from the outside as a good family means nothing. Grades don't prove or disprove intelligence, or disposition, it doesn't give us any information to go on. That you percieve his family life as good from the outside doesn't really mean anything either, what you would consider a good family, and what others would consider a good family is different, and your perceptions of it might differ from reality.
With that out of the way, it is rarely seen as the only solution, but often the only bearable one. Clinical depression is rough, it get's really bad. Bipolar is even worse. A burnout is similar, in ways. Often a sense of perspective is lost, and consequences get exaggerated. The slightest, smallest error, or thing that goes wrong can feel like the end of the world. I've experienced it myself, nothing seems to work, everything just piles up, and that tiny, small, little thing just gets you. It could be as simple as someone saying you're wrong. I had a teacher who got that, she was suffering from a burnout, and while recovering returned to school for a single class to see how it went. A student contradicted her, not in a harmful manner, or anything, but it was enough that she broke down into tears.
The attention argument put up by TheGreatCoolEnergy is a surprisingly valid one, for something put in such a ridiculous way, as well, but in an entirely different way, and for entirely different reasons than those presented, of course. With depression, or a burnout, or whatever, you usually realise you have a problem, that you need help. But it's not something that's easy to talk about. It's usually not a conscious motivation, it's just your body's defense mechanisms way of making sure someone will notice, someone who can take better care of you than you can. Sometimes those mechanisms backfire, and take it too far (sort of like with a fever, it's designed to kill whatever ails you, but your body will keep heating you up until you break). That's what happens in suicide cases. I've been there myself, both with an attempted suicide or three, and other actions I can't later explain. I once threw a chandelier through my bedroom window. I wasn't angry, or anything like that, it just made sense at the time. I really needed help, and I couldn't communicate it, so that's what I did.
Yet another reason could be the fight, probably in combination with the other factors. With all that in mind, pressure, whatever ailments was there, whatever issues we have no knowledge of, perhaps the only person he truly reached out to was her, not to say he didn't have any other friends, but putting all your stakes on a single person, staking your very being on that person, it can seem very logical at times, and it's very easy to do. Getting into a fight with such a person could easily be the proverbial final haystack that broke the camel's back.
Also, I know this should be common sense, but don't listen to the insensitive assholes who don't know what they're talking about. I wish there was some way I could make people stop speaking out about things they have absolutely no idea about, especially in insensitive and rude ways, but unfortunately there isn't.
I know I sound a bit cold when I write this, but I'm trying to make an impartial assesment of the situation, to answer your question. My condolences to your girlfriend, you, and his family. I hope I've given you something to think about, I know blaming him, calling him stupid for taking his life is easy, and probably feels pretty good, but unless we understand these things, it will happen again, maybe not to you or anyone close to you again, if you're lucky, but people like some of the fuckjobs in this thread are causing a lot more problems than they could ever imagine with attitudes like the one's they have, even if it is just an act on the internet.
