It's a call to seize the moment, to begin a quest to make our own meaning out of life's meaninglessness, turn away from the things that hinder us, do what we know we should do, live how we know we should live, and be as we know we should be. Why? Because we only live once. It's our one and only shot.
I wish there were a literal resurrection and that life really were a journey towards an afterlife, but I don't have any reason to think it is. The Lenten journey -- from Ash Wednesday's brutal confrontation with life's fleeting nature, through the agony of Maundy Thursday and the death of Good Friday, to the resurrection of Easter Sunday -- is a journey from meaninglessness to meaning, from the bare bones of existence to a life that matters.