Why BRAZILIAN JIU JITSU? So many reasons, but some at the top of my list:
1) THERE IS NO BETTER ACTIVITY FOR GETTING INTO SHAPE. I was in cosmetic shape before returning to the mat but my conditioning is now there too. Whatever you're doing, it's not the equivalent of BJJ. Even if you have zero desire to spar, the classes themselves WILL get you into shape.
2) DEATH OF EGO. Nothing will kill your ego like getting smashed or twisted into a pretzel by a guy (or woman) you outweigh by several weight classes. It is literally impossible to progress in actual BJJ (sparring) without getting repeatedly destroyed. Most people walking the streets have no idea what could happen to them if they came up against someone with skills. People (let's be serious, males) tell themselves all sorts of lies about what they'd do in a fight, but in BJJ, there is only truth. You see what can happen, in the blink of an eye.
3) SENSE OF FULFILLMENT AS YOU PROGRESS. We're all bad (literally, not in the cool figurative sense) when as white belts we begin our journey, but I was particularly awful, with terrible instincts. I've not so jokingly said that I could have improved rapidly had I as a rule went against my instincts. If we were supposed to hip escape to the left, I'd invariably go to the right; if we were supposed to grab the same side lapel, I'd grab the opposite; and so on. Those white belt tests that nobody fails? I failed, as I could not complete a simple kimura. (I think Michael Fowler felt badly about not giving me a stripe, but I was incompetent, even for a white belt.) I have never faced a more difficult challenge than improving at BJJ, so when I did improve (slowly but surely), nothing felt better. I worked hard in law school (I don't think anyone worked harder than I did) but it quickly became apparent to me that working hard would lead to my doing well. I have less (or no) natural affinity for BJJ, so small steps in progress are more rewarding than any academic distinctions were. It took me over four years to earn my blue belt, and I was as proud of that as I was of anything else I'd accomplished in life.
4) CAMARADERIE. Your training partners are in a real sense FAMILY. You try to kill each other (normal people don't understand this at all), at times you might have even choked each other unconscious (hey Juan Manuel VA), but it's all in an effort to improve everyone's skills. Further, if you've never trained, you probably have a warped impression of the types of people who do train. You're way more likely to find nerdy types on the mat than you are to find meatheads.