It's basically because getting drunk is fun, it makes you drop your inhibitions, it makes everything funny, it makes you smart, witty, likeable, the life of the party (well, not REALLY, but that's how you feel while drunk).
Now the problem is that once you get to that point you've lost enough of your inhibitions that you can no longer tell when it's time to stop... so you get worse off than you should, vomit, pass out, hangover, etc.
So I guess people just sort of take the bad with the good.
Personally I don't drink very often at all because I enjoy my higher brain functions. However there are at least a few nights a year when I let loose and have a blast getting drunk.
I'm also one of those intolerable fucks who can drink anyone under the table, remember the entire night without a problem (all the better when you're the only one) and get no hangover whatsoever.
EDIT: The above would be the "logical" reason for drinking too much... the other one (and probably much closer to the OP's experience) is a sort of juvenile rejection of authority or something like it... Parents say you're not allowed to drink, so fuck that, we're 13, we can get booze because nobody really minds (very similar where I live) so let's drink 'till our livers drop. These are, let's face it, not the best representatives of the species.
I don't even think they even enjoy the alcohol that much, it's more about being "cool" and doing something they aren't supposed to. And everyone knows that smokers and drinkers are the epitome of coolness.