Why do men go through mid-life crisis?

It is common for males to experience a time of looking back at what they've done and looking forward to a shorter future of less physical ability. This can be a life changing period although not necessarily traumatic. This is often happening to men in their early fifties. For many though, this is also a period when issues that have been shelved for years and filed under "just get on with it", come out and need to be dealt with. This can cause severe depression, and debility. For me, this was a time that I had to accept my transvestism. I had tried to deny it for years but it is fairly well documented that this is impossible. Men are ( or more commonly, used to be) under great pressure to achieve, for family status, income etc and the self-perception of failure is something that many partners cannot help fight against.