Relationships are based on trust, and in order for a relationship to survive, trust must never be lost. This isn't only valid for romances, but also for friendships and family relationships.
Even when we interact with people that we don't know, we still rely on and need trust in order to feel comfortable. The more we trust somebody, the easier and faster we can open up.
At this point, we should remember how prone people are to making mistakes and hurting others. Therefore, we claim that the worst promise you can make to someone is 'I will never hurt you.'