Early in a relationship we see only the good things. Months later we start to notice bad habits. Years go by and those habits begin to annoy us. Before we know it, we dislike everything that we use to love. What is the difference between this and a friendship that falls apart? Unconditional love.
If unconditional love is missing, in any kind of relationship, then forgiveness is absent as well. If forgiveness is absent, there is no way for it to survive, because all of the things that are hated, become the things to be focused on. This is why it is so difficult to have intimate relationships with people. There is no (true) love.