Surely, when you ask one bully why he does it, he'll most likely answer, "because I'm bored and I've got nothing else to do, and I wanna have fun." That answer will almost always be true because more often than not, these bullies have got nothing good going for themselves, and their only idea of fun is at the expense of others.
Personally, it's amusing to think that whenever these bullies berate, insult, or push you around, they do it during those times when they feel pathetic and inadequate about themselves. Due to the simple fact that they're insecure and jealous, they have the constant need to bring someone down to validate their self-worth and assert that they're better than their victims.
Bullies will target anyone they think are inferior to them, but this is just a front to hide their own deficiencies. They waste their time to bother you because they want you to notice them, and they win when they get the attention they so crave from you. This kind of attention seeking only means that you matter to them, and you can win by paying them no attention, because that would make them realize that THEY DON'T MATTER TO YOU.
Then, there's the option to stand up for yourself. When a bully pushes you, push back and hold your ground. Don't let yourself be intimidated. Don't let them have their way with you, because if you allow them the first chance, they'll intimidate you every chance they get, so DON'T BE A PUSHOVER.
But here's the thing: you can choose to turn the other way, or you can choose to fight back; either way, YOU STILL WIN, because that's simply what bullies are--INSIGNIFICANT LOSERS.
At the end of the day, regardless of your social status or your supposed inferiority, you still win more in life than bullies do, because the first time the bully you, they've already lost. They'll never beat you, and they'll never BE you.