I think that one of the most difficult parts of life to relate to others in is time.
Throughout life, we go through countless different stages. Each stage of life brings with it its own set of limitations, chief among those often being time. One of the reasons people suddenly find it easier or harder to relate to others in general is when they've gone through the same situation; this is as true when it comes to time as it does to so many other aspects of life.
What is often difficult for a person is when they switch stages of life and the amount of time they have suddenly changes dramatically. A person who had plenty of busy time and suddenly find him or herself with a lot of free time can be overwhelmed, and start to feel down as they can't seem to fill the time. Another person might be thrilled to have all the free time. On the flip side, a person who had a nice amount of free time may find it hard to adjust to having so little - whether because they have so much to do in that free time as they're accustomed to, or because they simply aren't used to having so much busy time in general.
Beyond the person whose time has suddenly changed are those who know this person: Friends, family, other workers... When a person is used to their friend being in a certain stage of life with a certain amount of time available, they can take for granted their ability to call their friend up and talk, invite him or her out, or the like. When that changes, it can be as hard if not harder on a person's friends and family as it is on the person her or himself.
It's interesting to see this phenomenon repeat itself at so many stages of life: As people move from elementary school to high school, high school to college, college to the working world, single to dating to being engaged to getting married to having kids, from being employed to being unemployed to being employed again... You'll often find that people who have already gone through or are simultaneously going through those same changes understand that change in time on top of the other changes so much more easily. Those who have yet to really face those often have a much more difficult time adjusting.
How often does a person hear someone complain about a friend who suddenly seems to "disappear" when they get engaged? (I've done this complaining recently.) For the person who got engaged, they're simply unaccustomed to the sudden reduction in available time. For their friends, it's weird that their friend is suddenly unavailable. Neither side is "wrong" - it just is.