The human conditioning to look outside when in lack or need, automatically and very seamlessly attracts such relationships into our lives that seem to complete us and somehow make us feel lively. And conditioning by default carries an expectation that the other will fulfill for us that which we seek (in lack).
However, conditioning always forgets that it is in "The Other" we are seeking what we need and we never ever have complete control over "The Other"! So when people fail us in friendships and relationships, it is only because our expectation broke! The expectation that "The Other" couldn't meet or fulfill!