Since humor/sarcasm is like my 2nd language, clients often get this type of response from me when they tell me they're sore from a previous workout (seriously, they'll tell you. lol).
They also know that I don't advocate looking for wicked soreness as an indicator of a good workout, but it is an indicator that they were pushed out of their comfort zone a bit.
There are times when you'll just feel a little something while moving and think "oh yeah, I worked that the other day" and other times you dread sitting on the toilet, sneezing, walking up the stairs and/or twisting to get anything in life.
The latter shouldn't be the norm, you can easily put too much stress on your body and overtrain that way.
Muscle soreness is caused by your muscles reaction of the tearing or break down of muscle tissue caused by overuse through strength training, strenuous cardio and sometimes just active living in general that's outside of your norm.
While you can't avoid muscle soreness all together, here are a couple of ways to get over and reduce muscle soreness: