I have found that each and every one of his relationships with women during college conformed to the pattern of "friends with benefits" aka fuck buddies. Explanations for this phenomenon, which has been detected in the lives of most if not all of my close friends living on the campuses of undergraduate universities around the country, may vary.
Time. College is taxing, and it is often the case that people simply don't have time for other people. Example: my first major relationship in college lasted for all of a month. She was an overachiever par excellence - a freshman chemical engineering major with neurology aspirations carrying 18 credit hours of coursework at a 4.0 level, a part-time job as a microscopy technician, etc. I was forced to schedule dates nearly a week ahead of time. The breaking point was when I had planned a Saturday morning hiking excursion followed by Saturday evening dinner and a movie. She hurt her knee during her daily morning two-mile run, and then breathlessly informed me about two hours before we were supposed to leave that her presence was required at work that night, as well as having to finish a twelve-page research paper on nanotechnology by Sunday afternoon. We broke up by mutual agreement five days later. By contrast, it was far easier to maintain the relationships that followed, given that the only planning necessary was a determination of when the girl in question's roommate would be out of town.
Culture. Thanks in part to the successes of the feminist movement, and in part to the general open-mindedness found on most college campuses, it's not really that frowned upon to have an informal relationship with a member of the opposite sex. While the infamous gender-based double standard remains - a man is a player while a girl is a slut - people of this one's generation can engage in mostly uncommitted sexual relationships without fear of social ostracism.
Fear of commitment. Nobody in college wants to be tied down, lest one has already found someone with whom they feel they can spend the rest of their lives. Needless to say, this is rare. So most people are still looking, still keeping their options open. Having a more informal relationship can satisfy the natural human requirement for emotional and physical intimacy without locking one in to the undesirable strictures of a long-term committed relationship.
Emotional fragility. Love can suck. People might want to avoid it, at least until they get all of their psychological shit in one sock. Example: Not unsuprisingly, three out of the four women with whom I had engaged in such a "friends with benefits" arrangement had, within the past six months, terminated an intensive romantic entanglement with someone they had known since high school (in one case, middle school). They were burned out on love. Or they just got burned: cheated on, lied to, whatever. My experiences with what this one thought was love ended similarly, and I was thus reluctant to experiment any further with such a dangerous unknown quantity. I believed that these women had been subject to similar experiences, thus rendering them temporarily unwilling or unable to engage in any real "relationship."