I'm currently reading the new book about Bl. Mother Teresa "Come by my Light". It is a facinating look into this great saint. I am amazed with each page how much love was in this woman. She gave her life totally to Jesus and the fruits of that are felt around the world.
Why must we give ourselves fully to God? Because God has given Himself to us. If God who owes nothing to us is ready to impart to us no less than Himself, shall we answer with just a fraction of ourselves? To give ourselves fully to God is a means of receiving God Himself. I for God and God for me. I live for God and give up my own self, and in this way induce God to live for me. Therefore to possess God we must allow Him to possess our soul.
Explanation of the Original Constitutions of the Missionaries of Charity, written by Mother Teresa, updated, p.28