In a free society people are free to identify themselves as they please. They are free to do with their bodies as they please. I don't have to accept their choice, but I see little reason not to. They aren't harming me.
There are a few of people who are uncomfortable with their biological sex. In some cases, quirks of genetics might contribute to this, as people can have extra sex chromosomes. There are also a few cases where fetal development will manifest a different sex than their genetics. It doesn't particularly matter to me the reason someone wants to change their sex. I think that people should be able to do what makes them happy.