Repeating an action expecting a different result is one definition of insanity. This fits perfectly with workout routines that yield little success.
Just as you get bored doing the same thing repeatedly, like eating the same thing for breakfast everyday or watching the same movie every night, your body gets "bored" doing the same exercises over and over again. This drives people "insane" while working out, wondering why their favorite routine isn't getting them what they want.
This can work in a positive and negative way, using The SAID Principle.
The SAID Principle stands for Specific Adaptation to Imposed Demands. This means that the body will specifically adapt to the type of demand placed upon it. So, if you lift heavy weights, you can expect an increase of maximal strength. If you lift lighter weights for many reps, you can expect a higher level of endurance.
So, lets say your goal is to increase strength. You lift heavy weight expecting to increase your maximal strength. Over time, you get closer to this goal. Your body adapts to the weight, becoming more efficient at lifting the load. You become stronger as a result. But, now what? If you keep lifting the same way, nothing new happens. No additional increase in strength, no additional effort. It's just not the same, so you can't expect anything new to come from it.
Most people at this point would increase the weight, when in fact this is only one of many ways that increased stress can be placed on the body. There is more than one way to challenge your body, continuing the cycle of adaptation without increasing the risk of injury.
What many gym goer's don't know, is that your body is made up of different types of muscle fibers. By just increasing the load or weight, focusing on the power movers (main muscles), the stabilizing muscles are more prone to injury, leading you down the road to breakdown or exhaustion. Exhaustion is when the stress is too much for the body to handle causing stress fractures, muscle strains, joint pain and overall fatigue.
Training programs should provide a variety of intensities and stresses to optimize the adaptation of all different tissues to ensure the best result. By focusing on the body as being more than one dimension (more than just lifting heavy) you can use the SAID principle of adaptation to your benefit rather than your detriment, achieving a higher level health and fitness.