Principle of Overload

In order for your body to continually adapt, you must work the body beyond what it is already used to. This is the principle of overload. The lack of consideration for this principle is what slows, and even prevents progress in many people's training.

If you go from using a 0lb. dumbbell to using a 15lb. dumbbell, you are going to make some adaptations. But the body will soon have the capacity to work with an even heavier dumbbell than 15lbs. If at that point you continue to work with the 15lb. dumbbell, you will make no more progress.

Essentially, your body's capacity continues to go up as you get stronger and more fit. This is why you must work harder in order to achieve enough of a training effect (the more conditioned you become). If you don't continue to work to your full capacity, you won't continue to progress. In fact, the lack of stimulus can lead to regression

There are other ways than just increasing weight to overload the body. If you want to know other ways just ask. But what I ask of you is to get out of your comfort zone and keep trying to work harder than you did during your last work out.