Market Upswings: What's the downside?

Bull rushes are great when you have a bunch of money already IN the market. It really is a rush to see your assets grow without you really doing very much, especially when it happens the way you planned. In fact, sometimes, these are so good, they happen more quickly than you ever envisioned, and they happen a lot more quickly than you saw coming.

However, I think there is a tremendous downside to the market running upward.