One correlation I've long noticed but can't explain is toughness and cheapness. A high percentage of the tough guys I've known have been cheap. (The former often implies the latter, but the latter only rarely implies the former.)
I should distinguish between being frugal and being stingy: frugality means you're willing to go without luxuries yourself, stinginess that you're unwilling to be generous -- or even reciprocate -- with others. Obviously, the first trait is admirable, the second merely selfish and, sometimes, hypocritical. The tough guys I've known have been both frugal and stingy.
Sometimes these guys seems to have ridiculous attitudes, as in, "Why should I give that guy money just because he brought me my food? That's his job." This may be social naiveté, but can also be an extension of a chip-on-the-shoulder attitude which occasionally characterizes them.
Anyway, I can't explain it, but I've seen the correlation too many times not to be struck by it.