This unity and communion of love to which we are called is the way of existence of the Holy Trinity (see may signature, again).
For the sake of clarity, I want to add something more to this affirmation I made.
This way of existence to which we are called is, in fact, our true, authentic reality and nature. The model of love and unity of the Holy Trinity is not an outside given to which we must submit. It is our real existence! Whenever we egocentrically close ourselves, we sin, because we move from who we truly are. Sin doesn't allow you to love the other one, but only yourself, thus leading you to perdition.
There is this Greek word "perihoresis" to which I didn't find an English equivalent. I'll try to explain it. Perihoresis refers to the reciprocity between the Persons of the Holy Trinity, to their unity and also their "threeness" (I'm sorry, I am making up words, but I'm sure you all get me).
So, how does this perihoresis reflect in human existence? An example that comes to mind is the perihoresis between husband and wife (but it equally applies to the community of christians too). Man and woman become one body through their union in marriage. In other words, if someone hurts my husband, I must love him so much and be in him so much that I feel the person who hurt my husband actually hurt me. Or, if my husband experience joy I must experience his joy as if it was mine.
If you fail in something, I mustn't secretly enjoy that you failed (like a lot of times, unfortunately, we do); but I must feel your failure as if it was mine. And if you succeed in something, I mustn't feel sadness and envy that you succeeded. I must live your success as if it was mine. Your success is not a threat to me. We aren't in competition. We must promote one another.
For this kind of love... sacrifice is the key.