In No-Limit Texas Hold'em aggression is the key to success. Playing aggressive in position gives you so many options to win.
You can win three different ways when you bet. First off, you can bluff your opponent off the pot and make him fold his hand. You could have the best hand and have your opponent call with a weaker holding.
Finally, you could draw to the best hand. Your opponent may call you on the flop with a better hand. Even though your opponent has beaten you on the flop, you could hit your flush on the turn or river. There are so many ways to win when you bet as opposed to calling. There is only one way to win when you call.
Calling and showing down the best hand only gives you one way to win. Calling does not allow you to win a pot without a real hand. Many amateurs will make it a habit to call because they are too afraid to play weak hands.
In order to be a winning poker player, you have to play aggressive without a hand sometimes. All of the great cash game players have a hyper-aggressive style of play. At first it may be scary to put money into the pot without a hand. Over time, you will learn to trust yourself and use aggression to your advantage.
Take some chances and see how aggression can improve your game. Bluffing an opponent with nothing is more fun than winning with pocket aces.