I started 20+ years ago for health reasons and loved it. I don't think about why any more; I just do it.
Somebody posted this recently and I'm going to repeat it here because it's apropo to the discussion:
"A Zen teacher saw five of his students returning from the market, riding their bicycles. When they arrived at the monastery and had dismounted, the teacher asked the students, "Why are you riding your bicycles?"
The first student replied, "The bicycle is carrying the sack of potatoes. I am glad that I do not have to carry them on my back!" The teacher praised the first student, "You are a smart boy! When you grow old, you will not walk hunched over like I do."
The second student replied, "I love to watch the trees and fields pass by as I roll down the path!" The teacher commended the second student, "Your eyes are open, and you see the world."
The third student replied, "When I ride my bicycle, I am content to chant nam myoho renge kyo." The teacher gave praise to the third student, "Your mind will roll with the ease of a newly trued wheel."
The fourth student replied, "Riding my bicycle, I live in harmony with all sentient beings." The teacher was pleased, and said to the fourth student, "You are riding on the golden path of non-harming."
The fifth student replied, "I ride my bicycle to ride my bicycle." The teacher sat at the feet of the fifth student and said, "I am your student!"
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Keep the chain tight!