We think the reason we fast is self control, or to help the poor. It is true that fasting does help the poor, and requires self control. But these are not the reason we fast. To find out the reason we fast, we should turn to the words of the Son of God. While Christ was on the earth, his disciples did not fast, and we read:
JST Matthew 9:15 And while he was thus teaching, there came to him the disciples of John, saying, Why do we and the Pharisees fast oft but thy disciples fast not?
16 And Jesus said unto them, Can the children of the bride-chamber mourn, as long as the bridegroom is with them?
17 But the days will come, when the bridegroom shall be taken from them, and then shall they fast.
The reason we fast is in mourning that we do not have Christ living among us. When he is, then the Saints do not fast, even though the poor still need help, and we still need self control.
We fast because we are mourning that we have not proven worthy of his presence yet among us.