[JasonP530]

Take this as an example of why agression is necessary:

You are in middle position with KQo and raise and only the big blind defends. He is a decent thinking player, who plays fairly tight. The flop comes 258 and no suits, you bet and he checkraises you. It is likely that the flop missed him unless he has a small pocket pair(which probably hasnt hit a set, because of the agression, most people wait for the turn). He hopes you have missed and that he can make you fold. You cannot just call down, because your hand is very weak, and has little showdown potential, so your options are to fold(sometimes) or to reraise the flop, or raisethe turn. Either way, that puts a lot of pressure on the ace high hands and low pocket pairs he may hold.