Well there are catually many causes, but regarding lactic acid and delayed oset muscle soreness we can say that:
1. Lactic acid is an acid produced by the muscles when you are exercising in an intense way and that can cause fatigue or muscle soreness. Best athletes don't suffer from that.
2. delayed onset muscle soreness is a normal response and is part of an adaptation process, little bit hurtful, that leads to greater stamina and strength as the muscles recover.