G. Mark’s Answer
The best reason in my opinion is not what most people would say. Either to help people or to make money. I would state it in such a way as to explain why it's valuable to study psychology at all, regardless of what you plan to do with it. The idea is that, as I often say, we are social animals. Secondly, the trend in society is for projects to get bigger and more complicated and more ambitious. Therefore, you will have the greatest impact on society and on your own life as a member of a team of some sort. Understanding social dynamics and how people think is a very valuable talent to have. Studying psychology will help you do that. Of course, my own opinion is that psychology is fascinating. But others may not agree, at least at first. But human psychology is a tool that will help you maximize your impact and multiply your success. I've read and studied psychology, both at university and from "pop" psychology texts, and while I find them necessarily repetitive -- I mean, whenever something is correct, others have a high probability of agreeing and repeating the ideas, right -- they generally tend to be useful and enlightening.