I believe that a lot of it is that most people are not raised to be equiped to be in relationships the way humans are intended to be. Humans are social animals. We depend on each other for survival. If we were still cave men we'd live in small little tribes and everyone would have their job and we'd all take care of each other as a unit.
But society preaches "independance." At some levels, yes, we all must be independant. We must each be aware which berry's are poison and how to catch a fish if we get seperated from the group. But we are not meant to drag ourselves through the hell of life with no one to lean on.
Human contact and connection and love is actually a need. It is a proven fact that if you take a baby, and keep it clean and fed and basically "physically healthy," but you do not cuddle with the baby, or talk and play with it, the baby not thrive and die. So the drive to be with someone, to be loved and cared for, is actually a need as much as water or food. That's the nature side.
The problem is, on the nurture side, I think you're right, people do not know how to love properly. We do not teach proper love, respect, and caretaking of others. More and more society is geared toward "me." This is fun for me. This is interesting to me. This is mine. Look at the healthcare debate in the USA. So many people are not interested in the health and well being of others. Until the problem effects them, then they do not care. That's a massive scale.
But you bring that down to a personal level and I think you find the people who give their all and can accept their family/friends/partner for who those people are good and bad. But many more who only want to get and not give... or who don't want to deal with someone who has "issues."
I know that if my current relationship were to end, I am not interested in another one. I already have a very limited social circle concerning friends for the same reason. In health care they ask about your "support system" of friends and family. I would say probably the majority of people don't have one, or they think they do but when things go bad they learn the truth.