Why don't people let go of the past?

You only have one life to live, so why do many people waste it dwelling on the past. So many people have the mentality of "they have to pay for what they have done to us." Who is "they" and who is "us"? Why do people like to make the connection of the past to the present? The past only serves as a reference point to learn from, not to get revenge from. What good has revenge every done anybody anyways? As Ghandi said, an eye for an eye would leave the whole world blind. The point is this, why can't people focus on now, and stop worrying about the past? I realize that the past affects the present, and even the future, but dwelling on the past won't help you no matter how much you want it to. Stop wishing for revenge on people's ancestors.