Relationships do not have to be difficult. People have to know when to move on and deal with the issues that afflict them so that can learn from them, otherwise they would just be repeating them. One such mistake in relationship is continued infidelity when one of the parties feigns recognition and yet continues. It sounds like self-defeatist but there are plenty of guys that continue to play the field yet come on as being sincere. He may also be looking for a mother figure while she isn't looking for a parental role. Then there are girls who are so terribly attached to guys that they are either nauseatingly dependent on his every move or that they consider him to be their property. No one should belong to anybody else. The moment when one of two feel they are being used there should be some discussion to air out any misconceived notions or to resolve the situation.
I think for a relationship to succeed one has to have a certain level of maturity, not the same maturity as an older couple but a level of understanding and willingness to let the trivial stuff not get in the way of the harmony of the two living together. But I think that younger couples often disregard the work maturity thinking that is taboo or something that happens with middle age.
Some people test the waters to see if the relationship will hold up to snuff. Sometimes giving each other a certain liberty would allow the individuals to consider how well they feel another condition besides being together.