Solving problems that people never knew they had.
I go back to what Henry Ford said, when asked about his "horseless carriage": "If I had asked what people wanted, I'd have made a faster horse." I have done numerous user interviews over the years and have found that the real opportunity (and when the "magic" happens) is when you find an unmet need and then service it. The slight "eureka" moment that people have when you have given them a moment of satisfaction, and make their lives incrementally better is why I do what I do.
As a designer I can make people's lives better, and if it is only "better" by the slightest of degrees, it is still an improvement. To have a hand in bringing that about… well, it makes some of what I have to do in my job totally worth it.