I know it’s a cliche, but without it the relationship will not work. Many people forget how vital this element is in any relationship. Trust is not only associated with loyalty or infidelity. It’s all the meaning associated with the word: honesty, compliance, value. Trust is a vital survival skill. From an early age we learn the importance of trusting. We also learn the lessons when we erroneously put too much trust in someone. Trust with a loved one is a relationship of reliance. It’s a one-time agreement between you and your loved one. In good relationships, their trust is seen as full disclosure. There are no games or deceit. There is no conditional trust between them. It is there, it’s tangible and in a good relationship, it’s protected. - By Unknown 

And for me, it’s the trust that core of a relationship :)