Spoon's right, and it's usually pretty simple. It often doesn't have to be very detailed. Just something along the lines of "I raised the flop because I put him on a draw and thought I could get him to make a loose call" or "I played this hand out of position because I wanted to mix up my play" or "I don't think he makes that pre-flop raise with pocket 3's".

If you aren't able to do this, it may mean that you really don't understand why you played the hand the way you did. And if that's the case, you almost certainly played the hand incorrectly.