The answer is fully provided in the following sacred, or qudsi, Hadith which means that it is God's own words quoted by the Prophet (peace be upon him) but it is not part of the Qur'an: All actions done by a human being belong to him except fasting; it belongs to Me. I shall reward it accordingly.
The Hadith is longer than this, but only this part is in God's words. The remainder is in the Prophet's words, and it goes as follows: Fasting is a means of protection. When any of you are fasting, they must not engage in any quarrel or shouting. Should anyone verbally abuse you or fight you, you should only say: I am fasting'. By Him who holds Muhammad's soul in His hand, the mouth of a fasting person smells, in God's sight, better than musk. Fasting people rejoice twice: once when it is time to break the fast and once when they meet their Lord. (Related by Al-Bukhari)
The first statement in this Hadith, which is God's own statement, defines fasting as something a person undertakes purely for God's sake. There can be no pretense in fasting, because it is done out of one's free will. In another version of this Hadith, God is quoted as saying that the fasting person abstains from taking his food, drink and sexual desire for My sake. The fact that fasting is an act of abstention for God's sake is what makes it free from pretense and hypocrisy. The fasting person honors his commitment even when he is alone, unseen by anyone other than God.
A hypocrite who only pretends to believe in God will not fast. He will not be able to tolerate the pangs of hunger and the hard thirst. He will make sure to eat and drink when he is unobserved. Such purity of action and intention is what makes fasting what it really is, an act undertaken for God's sake.
Yet fasting is not merely abstention from food, drink and sex until sunset. It is an abstention from all things God has forbidden. It is not right that while one is fasting, which means that he is engaged in an act of worship, that one indulges in sin. That will put a terrible color on his behavior.
It is in consideration of all this that God treats fasting as a unique action by His servants. This is why he says that fasting belongs to Him while all other actions we do belong to us. These may have an element of pretense, or show off, or insincerity. Fasting does not. Therefore, God says that He rewards fasting on this basis, i.e. that it is devoted to Him. These words of God constitute a great promise.
The normal reward God grants for any good action is ten times its value. Yet God may increase this to any higher figure, up to seven hundred times. Fasting, however, has a special rate. It is not defined, because we cannot think of a figure that is worthy of being associated with God's gift. It is beyond our estimation.
When Muslims understand this, they realize that God has given them a chance to earn forgiveness of all past sins. In another Hadith the Prophet is quoted as saying: Those who fast the month of Ramadan, motivated by faith and in devotion, shall have all their past sins forgiven. (Related by Al-Bukhari) It is for this reason that Muslims welcome the month of Ramadan thinking of its blessings, rather than its hardship.