Some very aggressive people have DumbAggression habits (without which they feel their aggression numbers are too low). Betting when a flop misses them is a common type of dumb aggression.
If you are playing limit Hold 'em, you need a reason to bet the flop. The facts that nobody else has bet the flop and you raised pre-flop may not be enough. Now, if you can see your only two opponents fold too frequently, hammer away. Or if you have a back door flush draw as well as two strong overcards, hit them.
But if you have five opponents, many of whom hang to the river, and you have no back door draws, you have reasons not to bet -- any pair may take down your overs, and these are just the kind of people to hang in there to catch one. Or if you come in with AQs to see a king-high flop without your suit, all you have is one overcard. Even the weakest king has you by a wide margin. (And KQ will take you apart if you catch your queen.) Remember, after the flop, you are just beginning the second of four rounds of betting -- 75% of the betting action is yet to come. . . but you've already got 71% of the cards you're going to get. You can't bet when you've missed the flop without good reason.