You buy when it's on it's way up because you are panicing that you will miss out and you sell on the way down because you are panicing that you will get caught in a major loss (you've already held into a loss hoping it will turn back up). Both are actually very natural emotional responses to the circumstances but logically, if something has gone up a lot, then it doesn't have far to go before it stalls, and if something has fallen a lot, it has more potential to rise.