Trust is the most important factor in any and all relationships. It is impossible to have a lasting relationship without trust. When the lust is gone the trust has to remain, it is key to friendship and love. There cannot be love without trust, merely a strained connection.
That one person (or more) in your life that you find a lifelong attraction to is most likely the one that you trust even when you don't trust them. The arguments can be intense but in the end the devotion wins out and you find yourself still drawn to them in such a way that you cannot control it. It does not matter what the two of you have done to each other, you still defend each other to the ends of the earth.
Trust can mean a number of things and you can trust different people with different aspects of your life but trusting someone with your heart is an entirely different story. It requires that you be willing to let them in close enough to hurt you. Fear will prevent full disclosure outside of an area where confidentiality is mandatory as long as you are not confessing to a crime or the intent to harm someone.
Telling someone the secrets that you keep closest to your heart is a scary thing, but knowing they will never tell a soul is a feeling like no other. Trust is what leads to couples remaining friends long after the love affair has ended and makes everyone around them wonder why they are not together.
Trust is fragile, a single incident of not being able to believe someone has the ability to make sure that you never believe them again. The worst part is that there are those that do not realize that it can be the tiniest things that make it impossible for you to trust them.
Once you find someone that trusts you without reason, and that you can trust without reason make sure that you treasure them for the rest of your life. They are a rare find.