Calluses just seem to be a part of life. I used to get them on the ends of the fingers on my left hand due to playing guitar. I spoke to a man yesterday who said he used to have them on his hands from picking cotton. A lot of people have them from working shovels, rakes, or some other gardening tool. My guess is that from time to time you have developed some sort of callus. They develop from something being over-used or abused over time. Eventually the skin becomes hardened and the feeling is lost.