The word motivation stems from the Latin word "movere" which means to move. Your motivation level is what moves you to participate in an activity and it affects your desire to continue the activity.
Recently, exercise psychologists determined that identifying with the exercise is the primary motivator in beginning and continuing an exercise program. For example, a person who enjoys strength training makes it part of his/her life, trains regularly and feels good about what he/she is doing - he/she IDENTIFIES himself/herself as a weight trainer, a body builder or a strength trainer.
A secondary motivation is the GOAL. Losing weight, gaining endurance, gaining strength or winning a contest or award are all legitimate and common goals. A goal-oriented motivation is not considered as solid as an identification motivation, because once the short term goal is reached, people often become disinterested in their fitness programs.
Exercise itself can play an important role in helping a person feel "good" and healthy. When you are exercising regularly, you are much more likely to enjoy a feeling of well-being. So why do people stop exercising before they reach their goals or soon after reaching them?
The main reason people quit exercising is that they no longer find it "fun." To keep motivated you must really be clear in mind and body concerning the type of exercise program is right for you and your personality. And you must have several short-term goals. When you are not clear, you may quit when you first experience the mental and physical pain of exercise and instead of working through "The Wall", toward greater rewards, you are discouraged early. Can you visualize yourself as "a person who does aerobics?" As a runner? As a cyclist? If you can, your chances of sticking with your new exercise program are greatly enhanced.
Other smaller factors help too and can influence your motivation level. Such factors as time of day, freedom from other worries, ability to focus and general enjoyment are important. It is key to stay within your energy reserves, to focus inside yourself and to understand what is happening and why you are doing what you're doing. Evaluate and reevaluate goals, keeping them realistic, but challenging.
A final thought - exercise psychologists claim that if you can stick with your program for six continuous months, you have hurdled the biggest wall and are very likely to continue your program longterm.