I'm aware of that. I perfer to pay $10 for a stock that is on it's way to a new high of $20 and then moving up.
I don't want to pay $10 for a stock that used to be $20. Granted I would make the same amount with both stocks, but something occured that caused investors to reduce the perceived value of the stock to drop and that could happen again.
I'll leave the "If I liked the stock at 20, I'll like it more at 15 and love it at 10" to the value investor who is content to tie up their capital for a long time.