At the risk of sounding
evangelical..
Juggling is more than just tossing a load of stuff in the air and trying to catch it. It does not require special coordination - if you can catch a frisbee you can juggle. All it requires is an interest and an appreciation. Behind the throws, catches, bounces and the waving of the arms lie abstract patterns of various complexity. These range from the simplest 3 ball cascade to.. infinity.
The vast majority of jugglers are therefore inclined towards abstract pursuits, e.g. Mathematics, Physics, Computers and Music. They are intelligent with a high level of focus. When they juggle they can sense the patterns in time and space. This is why juggling photos usually look crap, i.e. they only capture a single instant in time. Also, jugglers who are using all their concentration on their pattern often don't have any left for composing their faces. For the same reason it is impossible to be stressed and juggling towards your limit at the same time. Like playing music, it demands all your attention. It's also a physical thing. It raises your heart rate and gets the blood flowing.
Learning to juggle gives most people a big buzz, especially if they saw it as some kind of rare magic beyond their capabilities. Progressing with juggling is a never-ending journey with a lot of lessons for life. It teaches you self-discipline tempered with patience. You have to accept there will always be better (and worse) jugglers than you, therefore you can only judge yourself against your own capabilities. It also teaches you that it's okay to drop, so long as each one nudges your juggling neural net. When you learn a new trick the immense satisfaction rewards the time spent working on it and the association between work and reward grows stronger.
And I haven't even started on the zen feeling of juggling eggs, hack mode, etc...