Why do some people's teeth turn yellow?

It is possible to have a tooth turn dark because there was no permanent tooth underneath. Mine was a baby tooth and all I was ever told by any dentist was, "You have a baby tooth, with no tooth under it, so I'd recommend leaving it alone unless it gives you problems." It remained and was a healthy tooth until my mid forties. At that time several teeth weakened but I was told that was due to chemotherapy and heavy medications, so the discolored or dark tooth may have lasted longer without the other complications.