Earlier quoted context omitted.
Totally agree but this is an unfortunately common behavior. During the dot-com boom many people picked up hugely expensive homes based upon the the perceived value of their shares. We all know how that turned out for most. Even now I hear of people buying homes way out of their means because they have some stock in the currently hot company of the week that has some assumed paper value.
How do you avoid these pitfalls? Just assume and act like you're still poor (or not rich) until you sell your stock?
Investors recognize there's a problem there, so there's a trend these days to find ways for startup employees to cash out a bit.
That, however, applies mainly to later rounds. The YouSendIt guys made classic rookie mistakes. They mistook startups for a get-rich-quick scheme, but they aren't; median time to exit for a startup is 7 years from funding. And that's if you're lucky enough to exit at all.