All right, so what is today's talk about then? It's about my childhood dreams and how I have achieved them. I've been very fortunate that way. How I believe I've been able to enable the dreams of others, and to some degree, lessons learned. I'm a professor, there should be some lessons learned and how you can use the stuff you hear today to achieve your dreams or enable the dreams of others. And as you get older, you may find that "enabling the dreams of others" thing is even more fun. —“The Last Lecture,”
Randy Pausch

According to this passage, why is Pausch giving the speech?

to tell his audience why it is important for them to achieve their dreams
to explain what he dreamed about as a child
to explain that he achieved his childhood dreams and to encourage others to do the same
to provide a humorous and entertaining story