When I first started volunteering at the University of Toronto, students occasionally asked me why I would teach for free. This was my answer in 2004:
When I was your age, I thought universities existed to teach people how to learn. Later, in grad school, I thought universities were about doing research and creating new knowledge. Now that I'm in my forties, though, I've realized that what we're really teaching you is how to take over the world, because you're going to have to one day whether you like it or not.
My parents are in their seventies. They don't run the world any more; it's people my age who pass laws, set interest rates, and make life-and-death decisions in hospitals. As scary as it may be, we've become the grownups.
Twenty years from now, though, we'll be heading for retirement and you will be in charge, because there won't be anyone else to do it. That may sound like a long time when you're nineteen, but let me tell you, take three breaths and it's gone. That's why we give you problems whose answers can't be cribbed from last year's notes. That's why we put you in situations where you have to figure out what needs to be done right now, what can be left for later, and what you can simply ignore. It's because if you don't start learning how to do these things now, you won't be ready to do them when you have to.
I updated my answer just before I left in 2010:
This is stirring stuff, but it isn't the whole story. I don't want my students to make the world a better place so that I can retire in comfort. I want them to make it a better place because it's the great adventure of our time. Two hundred years ago, almost every society in the world practiced slavery. A hundred years ago, when my grandmother was young, she wasn't legally a person in Canada. Fifty years ago, most of the world's people suffered under totalitarian rule; in the year I was born, judges could and did order electroshock therapy to "cure" homosexuals. Yes, there's still a lot wrong with the world, but look at how many more choices we have than our grandparents did. Look at how many more things we can know, and be, and enjoy. And most importantly, look at how many other people can too.
This didn't happen by chance. Every time we buy one brand of running shoe rather than another or take ten minutes to vote we are choosing one future over another. Every time we help someone do something they couldn't do before, we are giving them more choices and more control over their own life. The world doesn't get better on its own. It gets better because we make it better: penny by penny, vote by vote, and one lesson at a time.
I am less optimistic today than I was then. Climate change, mass extinction, surveillance capitalism, inequality on a scale we haven't seen in a century, the re-emergence of racist nationalism: my generation has watched it all happen and shrugged. The bill for our cowardice, lethargy, and greed won't come due 'til my daughter is grown, but it will come, and by the time it does there will be no easy solutions to these problems (and possibly no solutions at all).
So this is why I teach today: I'm angry. I'm angry because your sex and your color shouldn't count for more than how smart or honest or hard-working you are. I'm angry because the Internet has become a cesspool. I'm angry because billionaires are playing with rocket ships while the planet is melting and because Nazis are on the march once again. I'm angry, so I teach, because the world only gets better when we teach people how to make it better.