Are you a Tigger or an Eeyore? Randy Pausch's Last Lecture.

I cannot stress this to you enough. If you haven't seen this video, I promise you it is not 1 hour and 44 minutes wasted. I've watched it half a dozen times already, and I appreciate it just as much today as I did the first time I watched it some five months ago.

ETA: Get past the first few minutes of introductions... I swear to you, it's worth it.