Richard Grant at Confessions of a (Former) Lab Rat, discusses explaining to relatives and friends why it is he does what he does. He writes that while it's difficult for researchers to merely explain what it is that they do "intelligibly, to the questioner's satisfaction," but "I further suspect, in the biological sciences at least, the proportion who are able to answer the question 'why?' is much, much smaller."
Grant writes that that's because, contrary to public opinion — or even descriptions included in their grant applications — not all researchers are actively trying to cure disease. "My own mother had great hopes I'd cure cancer. For the longest time I didn't know where to even start to answer her," he muses.
It came as a relief, then, when he simply one day realized that he was doing science for the sake of science. "After this realization I would cheerfully tell friends and family that I wasn't trying to cure cancer, or anything else: like a philosopher in the pure pursuit of knowledge I just wanted to see how things — in this case the 'things' happened to be people — worked," he writes, adding that "finding out how things work is almost a definition of humanity."