Keep in mind there are not really out-of-bounds for skaters in the play. You can't run out of the endzone or jump over the scorers' table, but you're moving faster than you ever would in a "running" sport. The risk when you're skating full speed into the corner to battle for the puck is enormous. One miscue and you're possibly going headfirst into a wall at nearly 40 km/h. Every time you're without the puck, you're constantly making a decision on whether or not to try get it back or get in better position.

It's expected among all players that you won't try to win the puck at the expense of the other persons safety. What if your teammate is tripped, flies into the boards, and breaks his neck because of a reckless play by your opponent? A two minute penalty isn't going to smooth things over. So you enter a consensual fight with him. You make the opponent stand up for himself because your teammate wasn't able to defend himself earlier. You fight so that his whole team knows, if they start playing at the expense of your safety, they will have to answer. It sounds primitive and to be honest, kind of is. But we're all programmed to assess the risk of actions we take. If the only risk of sacrificing your opponents safety is a two (or if it's particulary bad, a five) minute penalty, you're probably going to make the risky play. But knowing there's someone coming knocking when you do, is a deterrent. Players who make these risky plays then refuse to fight are known as rats.

As a side point, fighting on ice is different than on land. When you're on ice and you punch someone, you're going to move backwards. It's different than having your cleat dug into the turf. Your skates will slide and reduce the impact on your opponent. An ice hockey fight is fundamentally different than a street fight. It's all self policing and respect for your opponents safety, not just punching faces.