You could say that it's all a matter of how enthusiastic you are in sharing your mutual agreement.
Handshake: It's a deal. (which is about as enthusiatic as I tend to get)
Bumping fists: Yeah man, that's cool. I'm with you on that one.
High five: All right! We did it!
Or in the case of pumping your fist, how enthusiastic you are in your own achievement.

Yeah, that works. But what if you are a loner like Jack? Shaking hands works, but you don't get emotionally involved with anyone. Therefore, a high five is akward, right?


KRW

How about this--Jack is =very= careful. He has to be. A high-5 is easier to turn into an attacking move than a handshake. Or would that level of paranoia be too much even for Jack?

How about this--Jack is =very= careful. He has to be. A high-5 is easier to turn into an attacking move than a handshake.

That makes a lot of sense. When you lift your arm your whole side is vulnerable, espesially if, like a hanshake you use the same arm/side as the other guy, when they would be able to get a blow in using their other fist.

Or would that level of paranoia be too much even for Jack?

No, I don't think so. Like you said, Jack is very careful.