I was reading through a brief article on behavioral psychology that provided a short list of reasons people feel motivated to engage in any activity. I have the sense that it is a bit superficial, and likely not comprehensive, but worthwhile to take note of as a starting point for deeper exploration.
The most basic motivation, too often assumed to be the only one, is intrinsic motivation: people engage in an activity to achieve a specific outcome that is the result of undertaking that activity. It's straightforward to the point of being a virtual tautology that "a person goes to the store to buy milk because they want to have milk," and in many instances there is no reason at all to consider any further motivation.
Another motivation which is commonly known is extrinsic motivation: people engage in an activity to gain something other than the outcome of that activity, which is generally granted to them by someone else. Anyone who has worked at an unpleasant job, which is likely everyone, recognizes that they do their work in order to gain a paycheck - and not for the love of the work or the pride of creating a product or providing a service.
Intrinsic motivation is a reward that arises by virtue of the activity, regardless of the functional outcome. For most people, hobbies and leisure activities fall into this category. A person who enjoys playing golf is not attempting to create or gain anything for participating in the activity, and quite often pays to participate in it, but finds the engagement so compelling that the activity becomes its own reward.
The motivation to engage in an activity to support self-perception departs from the realm of functional or tangible benefits and becomes entirely psychological: the reward of engaging in the activity is simply pride. Many people in highly-visible but low-paying positions are motivated by the need to perceive themselves as a member of their profession - even though there are more rewarding jobs available, they prefer to "be" a teacher, a soldier, a firefighter, etc. Self-perception is also evident in the way in which a person identifies themselves according to an activity: a person who enjoys playing video games calls himself a "gamer" and a person who enjoys surfing calls himself a "surfer" - this is a sign that self-perception is more important to them than the intrinsic rewards of being engaged.
Of particular importance: self-perception is differentiated from receiving esteem from others, though the two go hand in hand. If a person identifies themselves as a "surfer" in order to make a certain impression on other people, the esteem he receives from them is an extrinsic motivation.
Norming is similar to self-perception in that the activity that a person engages in is a characteristic of the person they wish to be, but is slightly different in that the activity is not critical to the role. A woman might buy a certain brand of peanut butter because she believes that "good mothers" purchase that brand for their children, or because it supports the sense of being environmentally conscious, or because it makes her conform to the purchasing behavior of a given social class. The distinction between norming, self-perception, and the extrinsic motivation of gaining esteem of others seem a bit hazy, as it would be difficult to imagine a situation in which norming is entirely unrelated to self-perception or esteem, or where self-perception and esteem are not interrelated.
The final motivation mentioned in the article was cognitive dissonance - the belief that a given action must be taken in order to put things right and restore balance. The author's example in this instance are all acts of civic service - cleaning up a park or painting over graffiti. The only commercial example that comes to mind is the moralist who purchases every copy of a book in order to destroy it, assuming himself to be protecting others from ideas of which he does not approve. But I sense that may be a stretch and that better examples might exist.
Again, this is a brief list taken from an article which I suspect is not exhaustive, but provides a good start at considering the various motivations that drive a person to take action. It is not by any means complete, sufficiently detailed, or perfect - but is likely a better alternative to resting on a tautology or allowing motivation to remain a mystery.