Mithra — like Tammuz, Adonis, Apollo and Horus — was a sun-god. So you see, the "reason for the season" is not the son of any god, but the sun up in the sky!
"I don't believe in Heaven and Hell. I don't know if I believe in God. All I know is that as an individual, I won't allow this life – the only thing I know to exist – to be wasted."