If you want to know why we call our God Father, listen to Moses: `Is he not your Father who created you, who made you and established you?' (Deuteronomy 32:6) 

Listen too to Isaiah: `O Lord, you are our Father; we are the clay, and you are the potter; we are all the work of your hand.' (Isaiah 64:8) Under prophetic inspiration Isaiah speaks plainly. God is our Father, not by nature, but by grace and by adoption. Paul too was a father: father of the Christians in Corinth. Not because he had begotten them according to the flesh, but because he had regenerated them according to the Spirit. 

Christ when his body was fastened to the cross saw Mary, his mother according to the flesh, and John, the disciple most dear to him, and said to John: `Behold your mother.' and to Mary: `Behold your son.' Christ called Mary John's mother, not because she had begotten him, but because she loved him. (John 19:26-27) Joseph too was called father of Christ, not as procreator in a physical sense, but as his guardian: he was to nourish and protect him. 

With greater reason God calls himself Father of human beings and wants to be called Father by us. What unspeakable generosity! He dwells in the heavens; we live on the earth. He has created the ages; we live in time. He holds the world in his hand; we are but grasshoppers on the face of the earth.