Well, crying out of sadness can be seen as 'realizing something is too heavy for one to take', which'd make a link to weakness... also, there's the link to men being warriors in the past, needed to be strong, never show any weaknesses and such, which might also been taken over into today's morality...
Personally, I consider myself as a very balanced person, which is why I hardly ever have such strong emotion swappings that I'm crying out of happiness or sadness, or whatever... being a martial artist, I consider being balanced as strong, which'd mean that crying - associated with not being balanced - would be weak... well, either way, I don't think someone else is weak if he or she cries...