Go here to read our co-founder and CEO Jess Lee's thoughts on why many startup founders are unhappy (but she's not, because she loves Polyvore :-).
Her basic premise is that happiness is the first derivative of success, so your really feel all the ups & downs that are a natural part of any startup. Even if you're on a general upwards trajectory, you still feel the downward momentum of the little bumps along the way.
The secret to staying happy is to have a great culture and great people. It's the glue that keeps things together through the downs. You should also remember where you came from and talk to other founders to get some perspective.
You can read the full post here.