Are you sharpening the blades from the bottom (flat side) of the blade? Blades that are sharpened on both sides (like a kitchen knife) will dull faster. It's a bad, bad, BAD shortcut. Not only that, but you cannot possibly ensure the blades are well balanced. Unbalanced blades cause excess vibration and shorten the life of a mower. The only way to properly balance the blades is to take them off the deck.
Blades should be sharpened at least twice a year. A dull blade will make grass shorter, but it isn't exactly cutting it. It's more like ripping it to shreds.
If your lawn looks brown a day or two after you mow, your blades are dull. Not only does it look terrible, but you are stressing the plant and also exposing your lawn to risk of fungal disease. Dull blades can also rip the grass (roots and all) right out of the soil.
A nice clean cut heals, whereas a ragged smashed blade of grass may not recover. Over time, dull blades just make the situation worse with every cut. I'm not saying you need razor sharp blades all the time, but there is a threshold. If your lawn is small, then the interval between sharpening is longer, you can get away with sharpening only 2x every year. If you have a 1/2 acre or more, then maybe every 6th cut check the blades. They should be at least as sharp as a butter knife.
Blades should be sharpened to a 45 degree angle, only on one side. The sharper the blade, the more healthy your lawn will be. Your grass will be spending it's energy on growing instead of healing and fighting off infections. Yes, lawns can get sick too.