Much of my working career has been in customer service, or that has been at least some aspect of my job. I enjoy the part of customer service that is the actual helping of customers, but have not enjoyed the part that is making more money for the large corporations that employed me. At the end of the day, my entire job performance, and by extension I, was judged on the company's ability to sell another widget and add another zero to their profit. If that zero wasn't reached, I was expendable. I would rather that my work help someone and not just sell them some widget that may only hold temporary value.
With personal training, I will be offering a service that will lead to a healthier customer. I will be able to share all the physical and emotional benefits of fitness that I have experienced over the last few years. I will help the customer lead a better life. This is something that can help them for the rest of their lives, not just temporarily. I can't imagine many other jobs/careers that can offer that as a motivating factor.