Soreness is still an unexplained phenomenon.
For a long time it was suggested that lactic acid was what created soreness. But that was then proven wrong.
Now, it appears that soreness is microscopic injury to muscle fiber. this type of injury then leads to bimechanical events, ending in inflamation and edema of the injured (sore) muscle(s). The events are slow to happen which is why the soreness doesn't occur until 24-48hrs. post workout.
It also appears that soreness seems to occur most when you change exercises, or change the way you perform a certain movement (IE switching the grip on your pull ups, or bench press). It seems like soreness is a result of a new movement pattern, which causes the muscle(s) to function in a different way. Perhaps, this creates the microscopic injuries to a different area of the muscle tissue?
But, some people don't get soreness. I really don't get sore....but, my training frequency is high. The more frequently you train, the more your body adapts, the less soreness you have. that doesn't mean that the workout was unsucessful.....you have to go by performance. if you are regressing, then something is going wrong in your training.