I love doing this too

I'm pretty competitive (even for a newbie), it sounds like most of the culprits in this thread are just being jerks. If someone passes me, I usually treat it as a wake up call and speed up.
I don't like it when I've passed a slower rider, then they pull up right in front of me at the next lights, so I have to leave them behind when it goes green .
If I get to some lights and there is a cyclist there before me I'll either pull up behind them or alongside to say hi. Then if they look slower than me I'll move off the lights quick, if they look faster than me I'll let em go first, then pass when I get a safe chance if they are going too slow.

But I enjoy nothing more than hunting down and dropping riders who are antisocial / dangerous. Usually the guys who run the reds and generally ride badly aren't hard to catch.