Buddhism and relationships
Buddha left his wife and infant-child to pursue enlightenment. This is not exactly the kind of example that will help a person gain a good relationship with his wife (or her husband). Within Buddhism having a good relationship can be seen as part of the lay values. For those not willing or able to pursue the vocation of a monk or nun, having a family and raising kids is a perfectly respectable way of life, though one is not expected to reach enlightenment in this birth if one does that.
Wasn't Buddha selfish to leave his wife and child?
By today's standards Siddharta, the bodhisattva, was certainly selfish to leave his wife and child. But by the standards of his time he probably wasn't, though he knew his father would be furious. For one thing: he would have known that his wife and child would be taken well care of by his father. Siddharta had lived a court-life, where the extended family joined together in one 'household'. Uncles and nephews play a large part in his biography and this is testimony to the fact that family life wasn't cut off at the level of merely a father, a mother and children.
So what can we learn from Buddha's life in our own relationships?
Buddha showed the world that we should not expect happiness to come from someone else. Even people who are married should not expect the other person to make them happy. Instead they should do whatever they can to make the relationship a happy one. If both partners do that, they will both be happier, because people in a happy relationship are happier than people alone (this is what psychology tells us). People in an unhappy relationship are also unhappier than they would be apart.
The vocation of nun or monk
Modern single life is a pretty lonely life, because our society is generally not made for people to not get married. There are few social institutions for single people. In Buddha's time, by contrast, it was pretty normal for men to roam the land looking for enlightenment. Groups of such men worked together and there were also teachers who taught other men (not many women that we know about) what they had learned in the area of meditation and mysticism.
Buddha's system of monks and nuns built on that, but included a social aspect. Monks living in the same locality were expected to take care of each other when sick and do a monthly ritual in which sins were admitted. They also decided on their local monastic rules together. [That is: they recited them and if changes were made without protest by the others, they were accepted].
This means that the monastery wasn't just a place for people to become enlightened, but also a place of refuge for people who were not willing or able to get married.