Tolerance and shared vision/values are definitely important. But that only comes after there is an equal desire to be in a long term relationship. For some people that desire wanes (for whatever reasons), and that's why these lasting connections (professionally and privately) are rare.
You nailed it. It's that unconditional mutual desire to maintain a relationship that is important. If you have it, almost nothing can break your relationship, and if you don't then almost anything can.
It's not terribly difficult to imagine how strong the bond between you and someone else is. Each relationship has a different strength to it, and they sometimes grow and weaken. Some can take a lot of strain and others can't take any.
