I like healing. There's a lot of personal joy / amusement to be had by pulling someone's fat out of the fire. I get great enjoyment out of saving younger players when they get a bad pull just as I happen to be running past. That, and it's fun to buff every player that runs past as I trot down a road.

For groups, I don't really care for the leader position. Having to know where to go, making sure everyone is following, giving directions, which mob to pull, etc. It's a lot of work and it's best done by a tank or scout class. I prefer a more passive role where I follow someone else's lead while making sure that everyone stays healthy. I can do the lead thingy, it's just not my cup of tea most nights. My group role is more about making sure that things are going well in the back while the tank / scout leads the front.

Then there's the god-complex when you have to make life/death decisions on the fly. It's a guilty pleasure. While I hate letting anyone die on my watch, I've been around in enough other games that I'm confident in my decisions. So I only feel guilty if the person died by accident. If it was on-purpose because I had other, more pressing, priorities or a lack of mana, then I let any complaints just roll off my back. But unless you're really an idiot, I probably won't let you die... but I may leave you at low health.

Lastly, it's really fun when it gets really crazy. That's where you earn the trust of your party members. When you get through a really bad pull, you're out of mana, everyone is at 10% health, but everyone lived... that's a good moment. It usually means that you judged aggro correctly, managed to put the right heals in the right place at the right time, and your party members start to think you're a psychic who can see the future.