Actually, it has nothing to do with dominance, but is a means of bonding and creating a form of support network. This may sound odd or counter-intuitive at first, but a man would never hit another guy on the shoulder in a similar manner if that man was a total stranger or if he did not have a close enough relationship with him. He would probably not hit his boss on the shoulder, unless they happen to actually be close, or the CEO of his company (assuming the CEO is a man, which is statistically likely). If it were about dominance, then we would see men doing this with guys they do not know, which we do not. It is only after they have gained a certain sense of camaraderie that they are able to hit another guy on the shoulder without it being problematic. Also, as someone else noted, homophobia is a huge regulatory force for straight men. If a guy hugs another guy or touches him in a way that a woman might touch another woman to show support, he is labeled gay for doing so. Gender non-conformity among men is largely equated to homosexuality, and so homophobia also functions as a means of policing gender norms. Transphobia, androphobia, and other gender related phobias can and do function in similar ways, but those are whole other issues.
