Being aggressive in poker is vital to becoming successful. Passive play may be rewarded from time to time but on the whole it will be the players who show aggression, by betting, who will be raking in the money at the end of the day.
The first and most obvious reason to bet is because you think you have the best hand. You find a pair of aces in the hole and make a bet as you want to win more money with what is certainly the best hand at the moment. But have you thought that by betting you not only do so for value but also for protection?
Say you hold a set of queens on a board reading 8KQ with two clubs. Your hand is currently only behind to a pair of kings but there are many turn cards that could see your opponent outdraw you. Clubs are the obvious danger for the flush draw but also any ace or nine could see someone with jack-ten fill their straight and potentially stack you. By betting here you are charging your opponent to draw and offering him incorrect odds to call you. This means you have more chance of winning the hand and forcing your opponent to fold.
Betting also gives you more ways to win a pot than simply calling. By calling the only way you can win the pot is for you to have the best hand. However, by betting you can get someone with a better hand then yours to fold, you can be called and still outdraw them to a better hand or be called and have the best hand anyway. It should be obvious by now that the merits of betting are far greater than those of playing passively.
One phrase I always try to remember is that betting when you shouldn't leads to you losing a bet, but not betting when you should can cost you the entire pot.
Even the best poker hands are worthless if they have been folded, so a little bit of aggression can pay dividends.