Have you ever stopped working out for about 2 weeks and suddenly go back to the gym? The very next day your body felt like it was hit by a truck. But over time you start to feel less soreness as your body gets accustomed to the stimulus.
What causes muscle soreness?
When you train and lift weight, your muscles are put under stress. Your muscles are not accustomed to the external stimulus caused by the force you are producing against the weights. Micro tears are taking place in your muscles which causes the soreness you feel.
After the micro tears happen in your body which is what people call "muscle damage", you feel the DOMS (Direct Onset Muscle Soreness) for 24 - 48 hours. However, over time, your body get used to the stimulus and your CNS (Centeral Nervous System) gets better at recruiting individual muscle fibers. The DOMS will start to lessen, but that doesn't mean you're not making progress any longer.