^Muzna, I don't assume it's always easy to meet someone new, etc. But njgal mentioned that the guy is going to an Ivy League grad school in a different state---grad school is definitely a place where you'll meet a lot of people with very similar interests and goals. And since it's an Ivy League, the rigorous atmosphere, etc., makes the social connection even more intense---the classmates will likely have a strong bond because of the shared pressure, and will have similar levels of ambition, successful past, etc.
Both partners are also young and good-looking njgal said, and in the US, being young, good-looking, successful, and having a job that brings you into a wider network of people will naturally introduce you to more opportunities to be unfaithful. If someone is dedicated to their morals and to being faithful, it's very easy to do so, but if someone doesn't have those values as strongly or is in a vulnerable state they'll be more likely to fall into temptation.