Why I Cycle: Understanding Endorphins From An Experiential Perspective
Look to other sources for a scientific point of view. What follows below is a purely subjective perspective.
Endorphins
Google Endorphins and you'll get a raft of info, so here's the short version: When you take heroin, or do more than a 25 minutes of physical activity, you release a substance into your body that creates a feeling of well being. That can be a good thing, in the latter case.
Getting into good enough shape where you can cycle for 25 minutes without feeling pain is another topic, but since I've been cycling for 26 years it's not something I understand any more. My body just knows how to do it.
It's also known as runner's high, which you can also google.
Good Enough Shape:
Ahah ! There is a trick to this whole exercise thing, and here's an easy test. Do a conversation with anyone, or with self, and see if the word "should" comes into the conversation. If it did, and it came from you, things are probably not framed right for a long term enjoyable experience. Never use the word "should" in a sentence with exercise in it.
Cycling is the kind of a sport where you don't have to be in such good shape. You just cycle more easily when you aren't feeling so chipper, and cycle harder when it feels right. That does something interesting - it puts you almost always in "good enough" shape. As long as you can get past that minimum 25 minutes where the dopamine kicks in - presto chang0 - you've got it.
Pain
Another rule I follow is the reverse of the no pain no gain rule. Save that for your son's football practice. Your body remembers pain, and it will remember not to ride if you give it too much pain. Save this kind of thinking for the nut cases who go to gyms 4 times a week.
Thoughts
I like to think a lot. Those who know me well might point out that some of my behaviors are awkward enough to make thinking a really good habit. I get to do a lot of that when I pumping pedals. It's good.