When you look at the historical rise of the market, we are almost always at the peak. 90% of the days from 1900 until today you would be right saying "prices are expensive, we are at the top" and back out. Only 3-5 times since then has there been a significant drawback.

I guess my thoughts are you could make that argument every day and there will come a day you are right, but you'd most likely be missing out on a lot of gains because, as the image shows, the market always goes up.