Trust

 What is the Most Important thing in a relationship to Sustain?

I don't know. But, for me, Trust is the Most important thing in a relationship. Honest to each other about the feelings. It is not just showing the love because of the obligation to the relationship. People don't have to show off because the Society is thinking in that way. In that case,  it will become so fragile that, in a slightest moment, it will break in the million pieces. It is better to talk the things out. It is always better to end a relationship before beginning of the second one in the same context. Let's say About a Husband and Wife in a Arranged marriage. If the Wife is not comfortable with the Husband, It is better to break it off a marriage than pretend love someone by ruining the life over it. It is better to talk to each other about the issue and sort out the issue. Look for the Solution if there is one. But i don't think there is any solution for the Cheating or being dishonest. The thing about the the Cheating is, It is never full.