As mentioned many times already, humans were made for humans - our purpose is to interact and help each other. It is part of our nature - it's what makes us human. As Marcus says, "it is our duty to do good to men and tolerate them."
He then goes on to mention that other humans can be obstacles to us (in action), but they do not impede our ability to have a good attitude. Furthermore, our mind - our rational nature - can turn these obstacles into advantages or lessons. It is this passage that Ryan Holiday based his popular book The Obstacle is the Way. His book is chock-full of stories about people who have used obstacles to their advantage.
A short, more modern-day cliche, way to put this idea is, "when life gives you lemons, make lemonade." This idea of the obstacle becoming the way is all about attitude. It's about making the most out of events that don't go as expected. Life is full of stories of people who seemingly failed at one endeavor, only to find that along they way, they found their way. Another book that is full of these types of examples, is Scott Adams' book How to Fail at Almost Everything and Still Win Big.
My advice to you is: start paying attention to successful peoples' stories. Almost always, you will find "failure" and "defeat" and those same failures and defeats become the the path that leads them to how they became successful.
(see also Citadel p. 197)