Trust.

To make a relationship successful, there must be trust in it. Without trust, a relationship will never remain for a long time.

Asking questions again and again and not believing in someone not only weaker the bond of love but at the same time torture the other person. If one person doubts in his/her mind, then ask just one-time with your lover, and if he/she gives you all possible proof about his/her innocence then don't try to torture other again and again because it hurts them so badly they think there is no love in the world, there is only pain nothing more.

IF you forgive someone's mistakes, then don't mention it, again and again, it only weakens the other person and forces them to make that mistake again...