Equal Justice Works posted a nice blog entry about the top reasons to be a public interest lawyer. Here is a truncated list:

#4: Responsibility–more responsibility earlier.
#6: Money–yes, money. A surprise to me but given the current climate, the playing field has become more and more leveled.
#9: Fun–You’re breaking down institutions and messing up the status quo against huge corporations (potentially). What isn’t exciting about that?
#3: Community–I know this is chronologically out of order, but it might be worth mentioning last. There there is a great and powerful sense of community among public interest lawyers. This helps drive their efforts and causes.