Why is it often easier to start a fitness programs then it is to maintain one?

I believe the answer is A, unless you have already solved it since this is a week old question. I'll explain nonetheless. 

When you start a fitness program, you are often very hyped that you are finally becoming healthy, going to lose weight, maintain your current weight. But, that hype does not last for very long. Overtime, you start to notice that the results from the exercise just are not showing up as fast as they used to and you feel less and less motivation to continue with that program. That is because your body gets used to the exercise you put it through and as that happens, your body creates a routine. This minimizes the results because your body is used to the program so it takes more effort and a longer time to see the effect of it. This causes many people to walk out because they believe that it was a failed attempt when, really, all they had to do was change up the regime every now and then.