Simple motivations are effective, especially as answers to the question of purpose.
If you tell someone "10 million people are affected by this phenomenon/event/law/etc.", not too much more work needs to be done in terms of building credibility. And yet such simple foundations offer up such opportunity to expand at length: who are the people affected? What kinds of effects are (or were) expected? What kinds of effects have been reported?
If you can state a premise in terms like this, it often serves as a really good foundation.
We can look at this as having a general structure: you take a population (e.g., "10 million people," "doctoral candidates," "pet owners," "left-handed men who are deaf in one ear and drink two cups of coffee and one glass of wine each day"), and you look at something that has affected or is expected to affect the population in question (e.g., a new law, the passing of Halley's comet, climate change, use of a drug, etc.).
This is extremely general, but it can serve as the motivating factor in many sorts of studies. It can be a history: "A report on the impact of Operations Research on farm workers." Now your reader may know nothing of what "operations research" is, but even so the structure of the sentence makes the general parameters of the project clear to anyone who knows what "farm workers" are. Now, of course, you go on to develop in greater detail your description of what "operations research" is, and how you are defining the population of farm workers in your study, but the basic foundation is there--something that the reader can use to refer to throughout the reading of the paper. You could put anything in there in place of "operations research"--"Halley's Comet," "Led Zeppelin," "Shakespeare"--and the sentence still reads. We might not expect Shakespeare to have had much effect on farm workers, but it doesn't mean that the effect can't be studied, and we wouldn't study it unless we had some reason to believe that it had had an effect. Of course, just as we can replace "operations research", we can replace "farm workers". We could, for example, replace "farm workers" with "pet owners." We could study--to combine some of the possibilities above--"The impact of Shakespeare on pet owners," which might be most relevant to frustrated dog owners: "Out, damn'd Spot!" Or we could, making yet another replacement have "The impact of a pet's death on pet owners." And that could be a real study for a psychologist--if it hasn't been done already.
This general sentence structure: "The impact of ___X___ on ___Y___" can be cast in past present and future tenses. It can be question or assertion. It can support empirical research, meta-analysis, history, suggestions for policy development. Depending on X and Y, the results of the study can be usefully generalized (e.g., "farm workers" might inform us about "non-unionized labor"), or further specified (e.g., "pet owners" might inform us about "widowed adult pet owners").
Simple motivations like this can really help on a personal level. It's very important to believe in the importance of your project. If you don't, getting through the dissertation is much harder. If you can state the purpose of your work simply, that can help make it easier to weather the storms of personal doubt, and doubt from those around you. If you have a clear statement of your question, then you can deal with accusations of frivolity. Either you can look back at your simple thesis and decide it is worthwhile, or you can decide that it is indeed frivolous. When you can anchor your work to a real population, it help give the work the weight you're looking for.
If you can state your motivating idea in this simple form it's something that you can rely on. This is especially true if you pick a population that you care about.