[Archangelsk]

Fear may be a reason, but one can't discount the competitive reason also. When a man sees an advesary, in this case - as stupid as it sounds - a woman who is trying to take over the pack, the primitive reptilian brain kicks in and does the flight or fight thing. The fight thing in this case is to deny what his competitor wants, and in this case - again as stupid as this sounds - what she wants has been programmed into her own version of the reptilian brain.

[Nick Danger]

Fear may be a reason, but one can't discount the competitive reason also.

Seconded. Men's lives are competition. They compete for women, for jobs, for everything they get in life. The last thing they want to come to home to, at the end of a long day, is another competitor.