Someone will say or do something that makes it evident that the relationship meant something different to them than it did you. When this happens you are actually feeling the loss of what you hoped the relationship would be. I call this "necessary loss." It feels like they are breaking your heart. This is because at some level your heart is breaking (and opening).
Now when I feel angry with someone or get my feelings hurt, I ask myself, "How is this different than how I hoped it would be?" This allows me to clearly see what the loss and sadness is- so I can let myself feel it and heal. By doing this, I no longer need to tantrum against the way they are or try to change them. After grieving the loss, I simply change the parameters of the relationship to make them more in sync with the reality of the situation, and go on my way.
So why on earth would I open myself up to this? Because it is a small price to pay for the love and beauty that can be shared with another.