Your muscles increase when you overload them with a stress that they are not used to. It is almost like they are saying, "Wow, I was not used to that weight, I better repair myself and add a little to get ready for the next time that happens." So your muscles are always in flux of being torn down and built back up, with the correct rest and nutrition of course.
In order to keep your muscles from becoming stagnant, and used to the weight, one major way to avoid this is to your progressive resistance, meaning to always increase your weights once you hit your desired rep range. If all you are doing is using the same weights day in and day out and you are just doing 10 reps and stopping b/c you hit the magic number, then your body is not going to change. You have to make it change.