1. Because it makes your life better. Don't get involved in activism to "save" other people; do it because working for progressive change will also make your life better (not to mention future generations). These struggles aren't abstract debates; they tangibly affect how we live our day-to-day lives.
2. Because doing meaningful work can be very fulfilling and life-affirming. The stress of jobs, personal drama and society can be put into proper perspective when you're engaged in something that really matters. On top of that, one person really can make a difference in this context; many activist organizations are fairly small, and new blood (even if it's just one person) can have a huge impact.
3. Because you'll be able to build up your personal and professional networks. Getting involved in any kind of activist movement will put you in contact with dozens or hundreds of dynamic, intelligent, interesting people.
4. Because times are tough and talk is cheap. Everyone has a political opinion, but how many people actually do something beyond voting once every few years? There are a lot of very urgent problems in our community, our nation and our world Don't talk about it, be about it.
5. Because collective organizing works, and is the only thing that has ever worked. Yes, we need to elect good politicians, create beautiful progressive art and read lots of books to make ourselves smarter. But all of those things, without people actively engaged in organizing, are powerless to truly change the world.
What other reasons are there? What compels you to do this work, aside from the fact that you're awesome? When we're feeling burnt-out or hopeless, what should we do?