One of my first teachers told me something that I didn't really understand until I got a little older - that there's a human need to have contact with with other humans. It's a human bonding type of thing. Guys like to box around with each other, kids like to wrestle around - you don't get that with a lot of other activities. Even if you found something that was mentally challenging and physically challenging, would it involve another person, or contact with another person? Even if you played tennis there's not that physical contact. A lot of people, boys especially, like contact sports. It's a primal urge. Martial arts give you an outlet where you get that kind of contact without actually having to hurt anybody.
I've heard thousands of people say this over the years. They come to work
out, they train hard and they spar and even if they've been banged up a
little bit, when they leave they always say, "I feel better than I did when
I came in." Whereas a person who didn't understand that would go, "You just
got your ass kicked, what do mean you feel better?" You've gotten that
contact and it's a good outlet. Martial arts always has that.