I've been doing a fair amount of Brazilian jiu-jitsu over the last eighteen months and I enjoy it. A lot. I like the fact that you can 'lose,' that is, get submitted, and start right over again. In boxing, losing generally involves a lot of pain. I also like the fact that BJJ doesn't give you brain damage.
What I like slightly less is the social atmosphere of BJJ. You always have to deal with other people. Classes are scheduled at regular times and even open mats call for a lot of social interaction. I don't like the structured times and places. In boxing you are alone much of the time, even in a crowded gym. Sparring is intimate but the gloves and headgear maintain a distance, not to mention the distrust and caution you need to show your sparring part.
I love people and hanging out but I've always needed to be alone a lot. Like the cafe, boxing allowed me to be alone around other people. Boxers almost always respect other boxers' privacy. If someone doesn't feel like talking, no one talks to him. You hit the bags alone. You jump rope alone. Roland Barthes called it a Jansenist sport. He was right.