Originally Posted by
Badhairday
When they get reanimated they get to choose between going back to the grave or accepting their new "life", that is the first quest you do. Since so many humans hated undead in their first life, I think it's safe to say that most choose to not die again based on human survival instinct, when you actually get reanimated you might even think that you finally have a second chance of living. From there it's pretty much humans themselves that forces the newly risen to join Sylvanas, not Sylvanas herself. No one but her accepts them, and since they don't want to off themselves they join her.
I read somewhere that most Forsaken go back to their old homes only to get forced out by their own families running with torches. I'm not, however, sure where I read that, so it might not be canon. If it is, it would explain their newfound hatred. They have a second chance at life and everyone they love hates them.
If they really wanted to die again after becoming undead, they would. The very first quest you do confirms this. Sylvanas isn't forcing anyone to stay undead, she simply reanimates them and asks if they think that's cool or if they want to go back to being dead.