Relationships - what makes them fickle?

Relationships, friendships....how fickle are they I wonder? How is it that someone you spent hours and hours talking about your life with, or enjoying the innocent but "oomph" moments that the company of another offers... all of a sudden those same two people don't even look at one another eye to eye, they grow distant, and choose to remain aloof and ignorant of the other person....almost like they choose to ignore the very person's existance. I guess there are several reasons for this, but usually the one reason I find as the root cause of most breaking relationships is the "breaking of trust". It may be a one way road, but one of the two people in the relationship see that the trust they had exerted in the other, the space that they made in their being to fit the other person in was handled improperly in some way. In some way, the other person turned out to be someone they "thought they weren't". As soon as a person starts feeling that they cannot decipher whether the other person is worth their trust, that's when the real break takes place. How important trust is for us that makes me realize.....how the building block of any kind of relationship is trust itself....once its foundations are broken, then the entire structure collapses.
Infact trust is itself the basis of any interaction with another person, trust is the very way we lead our day to day lives. We walk on the road with trust that a passing car will not hit us, that the person walking next to us will not pull out a gun, we travel in a plane with trust on the pilot, that he will take us through. Why without trust we cannot live at all........but the trust in everything else, comes only from one basic trust which is the trust in ourselves. If we trust ourselves, then we trust everyone around us....we forgive easily and we make relationships work. So that makes me think....that its the betrayal of trust that breaks a relationship, but its the trust in the universe that things happen for a reason, the faith that what another person did was also for a reason, maybe even beyond his own control....that things are as they were meant to be that makes even the most turbulent relationship come out strong....Never giving up on the other person........unlimited compassion...