One of the most common problems actually happens before there is a relationship. A lot of people don't take the time to figure out what they really want and need in a mate. Most relationship unions are fueled by "chemistry" or "happenstance".
We become "emotionally invested" with someone (before we truly get to know them). Later on we seek to change them or "help them" live up to their potential of being our perfect mate. We become frustrated when they can't or won't live up to (our expectations) and they become resentful of not being accepted for who they are. All of this stems from us not knowing what we want in the first place or not having the discipline to stick to our "shopping list" when it comes to selecting a mate.
Some people will say "communication" is the problem. However have you ever noticed there are no communication problems in the begining of a relationship? You would think the longer people are together the less problems they would have communicating! Many people confuse the words "communication" with "action". Communication is NOT an "ask and it shall be given" proposition. It's only one person expressing an idea or thought and having another person acknowledge they heard and understood what was expressed. Other items such as money, trust, and fidelity boil down to (finding someone who shares your same values!) One of the primary purposes of casual dating is for "due diligence". This is the time one uses to determine if the person they are seeing is someone they want to have an exclusive relationship with or possibly spend the rest of their lives with.
Whenever two people share the same values, want the same things out of a relationship, and agree on a strategy as to how to obtain them they usually are together for a very long time. Opposites may attract in the short run but it's like that attracts like in the long run!