Why has Christ commanded us to address God as Our Father?

That at the very beginning of our prayer he may awaken in us the child-like reverence and trust toward God which should be the motivation of our prayer, which is that God has become our father through Christ and will much less deny us what we ask him in faith than our human fathers will refuse us earthly things. 

Why is there added who art in heaven?

That we may have no earthly conception of the heavenly majesty of God, but that we may expect from his almighty power all things that are needed for body and soul. 

I. Expect Great Things from God

A. The best things we know from human experience point to even greater things from God. Or what man is there of you, whom if his son ask bread, will he give him a stone? Or if he ask a fish, will he give him a serpent? If ye then, being evil, know how to give good gifts unto your children, how much more shall your Father which is in heaven give good things to them that ask him?  (Matthew 7:9-11)

B. Good things come in life even though we are deeply and personally aware of evil.  The heart is deceitful above all things, and desperately wicked: who can know it?  (Jeremiah 17:9)

C. The Bible reveals God=s grace stretching across the generations.  And the LORD passed by before him, and proclaimed, The LORD, The LORD God, merciful and gracious, longsuffering, and abundant in goodness and truth,  Keeping mercy for thousands, forgiving iniquity and transgression and sin, and that will by no means clear the guilty; visiting the iniquity of the fathers upon the children, and upon the children's children, unto the third and to the fourth generation.  (Exodus 34:6-7)

D. God=s goodness to His people is as infinite as He is. But thou, O Lord, art a God full of compassion, and gracious, longsuffering, and plenteous in mercy and truth.  (Psalm 86:15)

E. God genuinely desires the best for His people.  For as the heaven is high above the earth, so great is his mercy toward them that fear him. As far as the east is from the west, so far hath he removed our transgressions from us. Like as a father pitieth his children, so the LORD pitieth them that fear him.  (Psalm 103:11-13)

F. We cannot reach beyond God=s good intent.  For as the heavens are higher than the earth, so are my ways higher than your ways, and my thoughts than your thoughts.  (Isaiah 55:9)

II. Do Not Let Earth Restrict Your Expectations

A. God is too large for cramped expectations.  Am I a God at hand, saith the LORD, and not a God afar off? Can any hide himself in secret places that I shall not see him? saith the LORD. Do not I fill heaven and earth? saith the LORD.  (Jeremiah 23:23-24)

B. It is impossible to expect too much from the Lord of Grace. God that made the world and all things therein, seeing that he is Lord of heaven and earth, dwelleth not in temples made with hands;    Neither is worshipped with men's hands, as though he needed any thing, seeing he giveth to all life, and breath, and all things;  (Acts 17:24-25)                      

C. All created reality serves the revelation of grace and displays a compounding testimony of God=s benevolence toward His people.   God, who at sundry times and in divers manners spake in time past unto the fathers by the prophets,  Hath in these last days spoken unto us by his Son, whom he hath appointed heir of all things, by whom also he made the worlds;  (Hebrews 1:1,2)

III. Pray by the Standard of Grace

A. Measure your prayers by your experience with God as savior.  He that spared not his own Son, but delivered him up for us all, how shall he not with him also freely give us all things?  (Romans 8:32)

1. God did not save us to think small and look for little things from Him. For ye have not received the spirit of bondage again to fear; but ye have received the Spirit of adoption, whereby we cry, Abba, Father. The Spirit itself beareth witness with our spirit, that we are the children of God:  (Romans 8:15-16)