Men are biologically hardwired to attempt to fight, mog, kill, compete with, and assert dominance over other males.

As a man every facet of life is a ruthless competition with other men and there is no escaping it.

In social circles guys try to "cock block" each other". They'll sabotage each other's relationships (seen it happen a million times before). Some will be a little more subtle in establishing dominance over other guys by trying to make someone the butt of jokes, especially when a woman is present.

In the workplace men snitch on each other, abuse their power to fuck over guys below them, and when it comes time for layoffs, hours cuts, or promotions, they'll get extremely cut-throat in their competitiveness. Even minimum wage jobs are like this.

In any conflict that involves a man and a woman, all men will blindly side with the woman no matter how much evidence is in his favor.

In the military guys get very competitive over promotions and will degrade anyone so much as one rank lower.

Homeless guys fight over territory and rob each other over what little they have.

The vast majority of murders involve one male murdering another.

The list goes on and on.

This is all instinct driven. Camaraderie among males is few and far between. Even in situations where all parties would benefit from cooperation and loyalty, men will usually screw each other over if it will get them a slight benefit. (See: prisoner's dilemma)