This is a fundamental question I am now going to try to answer. And at the same time encourage you to do the same!
How it started
I have to confess: when I was seven or eight years old I didn't eat the apple I was supposed to be eating. Instead, I went outside, dug a small hole and threw in the apple. It probably just rotted, but I liked to think that I have planted an apple tree. We've moved away quite some time ago and all there is now - I guess - is grass and weeds.
But this curiosity inside me, has led and is leading to many experiments - and a garden is one of the places where you can do lots of stuff without ruining anything (in the most cases ;)). That's why I have started to plant stuff. Experiment with what works and what doesn't and trying to find solutions to problems I encounter on my way.
In addition to that, a garden is THE place for partying, BBQs and relaxing. And if your garden looks great and even yields fruit or vegetables you can eat (!), your neighbors and friends will be impressed. I find relaxation not only in being in the garden, but also when I plant, harvest and do all the other stuff one has to do to keep up a garden. If you have a certain drive, it won't feel like work.
That's why gardening sprouted (ha!) from my curiosity and ended up being one of my favorite activities to get rid of stress. Meditative - Try it!