Motivation is a word we use to describe a certain kind of action-oriented behavior, or to give a reason for behavioral action. What causes a person to do something in repetition, what compels them from inside their mind, is how we define "motivation."
One way to think about motivation is to first think about "what" motivation is - its sources - and then explore "how" motivation works - its process.
Motivation is driven by behavior. It includes responses to one's environment or circumstances, biological and emotional forces, social interactions (or a lack in social activity), and cognitive actions. All of those things cause a series of reactions inside of us that drive how we act.
The interesting thing about "motivation" is that desire is not enough to maintain it.
For starters, you need to decide that you want to do something. Your decision to take action is what activates motivation. There are days when I want to go run, but decide to stay inside and work. I know my body needs the exercise, but I decide to work on a project instead. Often times, we hear these inner voices in our bodies telling us we need to do things - but until we decide to do - we are lacking motivation.
Another key ingredient to motivation is persistence. Once you decide to start doing something, you have to keep doing it until the goal is reached. I think a lot of people believe that "motivation" is some magical power that fills our body to allow us to accomplish more than we normally do. That speaks more to the "pleasure" we receive from success. But what happens when not everything is pleasurable? For example, running can cause pain or discomfort. However, persistence is what allows us to continue to achieving our goal or completing a task, even when things are not as enjoyable.
This brings us to our level of intensity, as well as a recognition of how we can use "pleasure vs. pain" to stay motivated.
Our intensity with a project is measured by our concentration and energy and effort. We all know people who bring a high level of intensity to their work. We have all brought a high level of intensity to a project or moment. Likewise, we have all had moments when we have been very relaxed or passive in our work - lacking intensity.
While I do not want to over-simplify the concept of intensity, I do think there is a strong correlation to the "pleasure" vs "pain" centers in our brain. We are genuinely more interested in things that we find pleasurable. It is the reason why so many people procrastinate (lack motivation) with their taxes, doing yard work, exercising, changing their diet, etc.