It seems to me that each case is unique, but speaking generally, I think that the main reason is when people do not come together for true love. People often do not know what true love is, and many do not know about this all their lives. They may feel affection or dependence on a partner, but not love. They may misunderstand what this means at all. People may like the appearance of another person, but not everyone can feel the soul of a person. Appearance disappears, and with the soul of a person you have to live your whole life. Unfortunately, this happens very often when people choose the wrong person from the very beginning.