Your motivation to exercise evaporates for the same reason many believe morality has a religious basis
Motivation can be a quirky thing. It often comes out of nowhere and, much to our disappointment, evaporates just as capriciously.
How many exercise or weight loss regimens have you begun only to run out of steam a month or two down the line? Do you remember what went wrong?
You began for your own personal reasons with little thought of what you might get in exchange. The activity itself or satisfaction it created was its own reward.
Then, something happened. You started seeing results in the mirror or on the scale. Perhaps you decided to try for someone. Without realizing, your internal motivation was displaced. Now, you were working out to get the attention of that person or reduce the number on the scale. That wouldn't be a bad thing, but the human motivation system has a significant failing we rarely take into account.
In 1976, Greene, Sternberg, and Lepper captured and documented the precise cause of your workout failings by playing math games with children. At first, the kids seemed to like solving the problems on their own merit. Then came the money. Once experimenters stopped doling out tangible rewards, the children lost interest altogether.
The accepted interpretation is that external motivation easily displaces internal, regardless of the latter's strength. And once the external motivator stops giving feedback, the initial does not return. Money was able to "kick out" the children's initial internal desire to play the mathematical games—permanently.
Possibly of more contextual interest to the readers of this essay, this is also the mechanism that drives the eternal struggle of keeping a public diary—an obvious oxymoron to anyone unfamiliar with this web site. How does one write for oneself (internal motivation) while simultaneously making it available for public consumption (external motivation)? It is such a difficult task that diarists must regularly make "psych out" entries telling themselves that they truly are writing for themselves.
Oddly, the same phenomenon that thwarts your workout longevity, the aim of your online diary, and the children's love of math games is why many suggest religion is the basis of morality.
Evidence suggests those raised without religion are at least as moral as those with. They're incarcerated at a significantly lower ratio than their populations. However, the religious widely believe that without a magical list of don'ts backed by the threat of fire and brimstone from above, the populace would break down into violent chaos. Applying the template above, we can easily see how one reaches this conclusion.
The 10% of the world who do not believe in a higher power and so are not threatened by it do not seem to be causing even their share of trouble, strongly suggesting that there is a natural human penchant toward what the religious would call good. We'll call this the internal motivation to be good. The external is obviously the threat of hell or reward of heaven.
While this simply answers why the religious believe morality requires threat/reward—because their own internal motivation to be good has been displaced—a far more disturbing question is posed. Is it possible that religious morality's external threat can displace our own natural goodness permanently? If that is the case, religion behaves much like a lifelong poison. However, if the displacement is not permanent, under what conditions can our natural morality return and how can this be facilitated?