i'm not sure fun or enjoyable cut it. i think it's because you get a sense of freedom, away from the rat race (even on the commute), a chance to clear your mind and awaken your senses with sight, sound and smells and, as said, the unconscious memories outweigh those painful ones.
I still remember the sunset i saw coming out of Meriden one day. I was already really enjoying my ride (commute home), it was cold on my face but i was snug in my thermals, it was late autumn and the sun was setting and i was feeling very "at one with nature". As i came out of Meriden and down the hill, what i saw was such a fantastic sight I actually stopped the bike to take it in. Swirly oranges, reds and yellows filled the sky like the swirls of icecream you see on the Walls' advert (the sun had just set behind Birmingham city, you couldn't actually see it). Leaves on trees were burnt orange, brown and red, green fields and city lights far off in the distance looked like sprinkled glitter. Not only that, but there was a smell of bonfires and barbeques in the air. The whole package took my breath away. i never thought i'd look at towards Birmingham City Centre and see something so beautiful (most the way the sky was and the glitter effect) and i hope i see again one day, but unlikely that all those factors will come together just as i ride over the hill... but if i do ever see again i hope i have a camera on me.