This morning I was participating to a workshop when a debate started about why making a startup. The best answer I know is from Guy kawasaki:

A presentation by Guy Kawasaki for the Stanford Technology Ventures Program Educators Corner in the School of Engineering at Stanford University. October 20, 2004.

Guy Kawasaki is among other things the author of a great book, the Art of Start

An example of his greate advice below is how to make a 10-slide great presentation of a company pitch:
Art of Start – Kawasaki