One of the things that separates good fantasy football managers from great ones is patience. We play a week to week game, which means we've got a lot of time to just...think. We've got seven days to make moves and trade for players before another set of games is played.
As a result, it's easy to get anxious and want to shake things up after one of your running backs comes through with another down week, even if it doesn't make sense to do so. You're often tempted to trade an underperforming receiver only because you're sick of that player, not because it's the logical thing to do.