In some old 60's cop movie the main character played by Richard Widmark was kvetching about the lousy life of a cop, and his new girlfriend asked him, so why he was still a cop, and he looked a little puzzled as if he didn't understand the question and after mulling it over for 1-2 seconds he said matter-of-factly "It's what I do".
I probably started commuting for some noble or practical reason, but now I just do it because I do it.