Companies fail because nobody wants to use their product. The process of going from a product nobody wants to one lots of people want is difficult and fraught with uncertainty; most startups fail to make this leap, which is miserable for most of the employees involved. Conversely, if you love a product, chances are there are many people just like you who also love it or will love it once they hear about it. Passionate users = growing userbase = more capital or revenue = your stock options will actually be worth something.
(Disclaimer: I worked for two startups that totally flopped; in both cases, I had no idea what they actually did until I started. Then eventually I said "fuck this getting rich shit", went to work for Google - which wasn't really a startup at the time, but I loved their products - and ended up making a lot more than any realistic startup would've gotten me. YMMV, but this seems to be a pretty decent heuristic.)