Perhaps a slightly more academic answer might be to read Sean Maloney's writings on the myth of peacekeeping. There are other articles circulating that touch on why Canada goes to such places as Afghanistan. It is an important discussion that more people need to participate in.
One point though, Pike's question is not one that should be merely blown off; it is fair to have a measured, reasonable response to it. My own answer to the question is simplistic, but it here it is:
We are in Afghanistan because it is in our national interest to be there. Our presence there supports our military and political alliances; it is tangible evidence that Canada is a participant in stopping religious and ethnic violence, and it is a commitment of our country to show that we stand against the religious fundamentalism and economic frustration that has contributed to terrorist attacks around the world. In short, we have soldiers in Afghanistan because we need to have them there to protect our own way of life.
Thanks for being there for us, troops!