As for me, juggling combines so many different things I like into one! Different reasons have been important in different times of my life, yet always there seem to be new reasons popping up that keep juggling interesting for me!
The most important ones:
Instant gamification! Goals are easily set and achieved
Enough physical effort to be able to zone out and not do other things at the same time, but not too much effort to make me tired after an hour.
Requires a concentration that will get me in the flow. When the concentration is not there, you will be punished
A community that I love! I could not imagine life without juggling festivals and clubs. I have no friends who are not in some way related to juggling or circus, and though I've met and befriended hundreds of acrobats, somehow I still connect better to jugglers.
If freerunning or chess or breakdancing or swimming community had embraced me, I might have become passionate about one of those. Yet it was the juggling community that came at the right time and where I had a strong connection from the start!
The ability to create. I like to make things.
The ability to stand out. I like the weird and the unusual.
The ability to perform. I like being the center of attention.
There are many more reasons, but I think these ase the most important ones. Many have overlap in other areas, which resulted in me being interested in many other circus disciplines and artforms. I regularily practise different forms of acrobatics, different genres of object manipulation, playing music and singing, performing circus, drawing and animating, and I have done all of the above also 'professional'. Juggling still stands out though