For me, I've always loved nature, outdoors, and especially trees.
As a child, who didn't know a spruce from a sycamore, I was always in awe of how many different types of trees exist, and their towering size (most of the time).
I have also always been an artistic person. I used to draw a lot, but drawing was never scratching the itch for me, neither did clay sculpting, or music, or pretty much anything.
Now, bonsai! Bonsai is easily the most dynamic and demanding art-form I can think of....this is not why I started, (started due to seeing the utter beauty of large Japanese azalea bonsais) but it is the one thing that does scratch the artistic expression itch for me.....scratches it real good.
Then when you start talking about making pots, and scrolls, and stands, and benches, and all the rest of it, the fun never ends!
Side note....:
It's easier for me to understand the price tag of a multi-generation old bonsai that is not only still alive, but thriving and getting better each year, as opposed to the price tag of a super pricey painting, or sculpture.
The time, patience, skill, and endless possibilities are all why I love it.
Edit: I also love anything that makes people cock their heads to the side like a confused puppy, and go, "huh?, wow!".