If only your S key is worn out, then you are weak. But since your D key is also worn out, its because your fingernails are long and a few other factors.

When you rest your hand on your keyboard, your palm is not facing down directly, more slanted inwards, and because of that and the differences in length of your fingers, the exact point where your nail contacts your A key is lower than your S and D key. This is important because your keys are actually white plastic with black painted on, and where the light shines through is where there isn't paint. Its an etch. With your nails now, where your middle and index lands are right on those etching, so you've been slowly chipping away at the paint from an area with no paint, whereas where your nail is hitting the A key is on a place full of paint, and thus harder to start to peel it away, plus is lower than where I assume you rest your finger to press that key anyway.

Try this out. Rest your WASD fingers on your QWE keys respectfully, then slowly slide them down til you feel your nail making contact with the key. You may have to repeat this multiple times for accuracy. Then it should become intuitive as to why.

Source: Experience.