Motivation has come up several times for me this week, and I have been able to talk about it and think about it in a number of contexts. Motivation is often an explanation for why someone is achieving or not achieving something. The parent who wants his adult-child to move out of the house is often complaining of a lack of motivation. The person who achieves an advanced degree can easily be said to possess a high level of motivation. It's a likely explanation for achieving something hard, or not achieving something that appears simple.
The problem with motivation as an explanation is that it is often used either in an overly simplistic way or trait-based (i.e., something you are born with) way. When parents start with, "If he would just...", and fill in the blank with "get a job", "study more", or some other "simple" solution, the solution falls short is because the situation is being over-simplified. "If she would just..." is my cue that the parent (or other frustrated party) is oversimplifying the problem, and will thus not look for an adequate solution.
On the other hand, people can feel in awe of people who seem to be "motivated individuals". They look at this person's accomplishments, amount of energy, or particular skills and think that motivation is to be credited for such. Motivation certainly does account for achievement in many situations, but motivation is also not something that some people are endowed with (i.e., trait-based, either you have it or you don't) and others not. We are all able to be motivated, and able to be unmotivated. In most cases, motivation is something that is strategically fostered and developed.
When we see people who appear to be under-motivated, often we are witnessing someone encourtering a barrier, or using ineffective rewards. In the former, "If he would just get a job," assumes that person knows how to get a job. It also assumes that the logistics have been worked out, such as transportation, specific skill requirements, and basic understanding of how to work and be productive in a way that someone else (e.g., the boss) finds acceptable. Tons of training goes in to the task of getting even the most meanial job. It should noted, too, that very little of our formal education (K-12) focuses specifically on "getting a job" as a thing.
In the latter, "If he would just get a job," assumes that getting a job is better than not getting a job (or, keeping things the way they are). When I mention this to parents, the first place they invariably go is the avoidance of punishment route. "Well, if he doesn't get a job, he's out on his ear." For some animals, this can be an effective motivator. For humans, this "reward" system tends to make the most sense to 10-13 year olds (see Kholberg's Stages of Moral Development). For adult-children, however, it might be more effective to talk about actual rewards, such as increased freedom and choice.
Most people are amazed at the near-magical appearance of motivation that occurs when realistic and meaningful goals and rewards are peppered into a frustrating, low motivation situation. Still others will see the idea of motivation arising out of managing goals and rewards as being too "soft" on people. These people like to tell me stories of when they were kids, and they had to deliver papers in the snow with no shoes before school, and how that was good enough for them. However you look at, ask youself if your expectations are being met, and the methods you use to get them met (e.g., complaining, criticizing, shaming, etc.) are working for you.