If I understand muscle training theory correctly, you put stress on your muscles, often times going as far as microscopically tearing the muscles up, and then expect the recovery process to make you stronger. Please excuse the gross oversimplification.
Muscle fibre tears can result in pain, otherwise known as Delayed Onset Muscle Soreness. More stress results in more tears, and more pain.
Would there be a relation between the amount of stress put on a muscle and the physiological reaction that rebuilds the muscle stronger afterwards? In other words, when I feel great soreness, should tell myself "well, this is a good sign", or rather "oh grief, I've gone too far"?