... outwardly the Fast is difficult and toilsome,

Even though outwardly the Fast is difficult and toilsome, yet inwardly it is bounty and tranquillity. Purification and training are conditioned and dependent only on such rigorous exercises as are in accord with the Book of God and sanctioned by Divine law… Whatsoever God hath revealed is beloved of the soul.
Bahá’u’lláh, The Importance of Obligatory Prayer and Fasting, XVI

 `Abdu’l-Bahá also explained:
The mind and spirit of man advance when he is tried by suffering. The more the ground is ploughed the better the seed will grow, the better the harvest will be. Just as the plough furrows the earth deeply, purifying it of weeds and thistles, so suffering and tribulation free man from the petty affairs of this worldly life until he arrives at a state of complete detachment. His attitude in this world will be that of divine happiness. Man is, so to speak, unripe: the heat of the fire of suffering will mature him. Look back to the times past and you will find that the greatest men have suffered most.
Selections from the Writings of ‘Abdu’l-Bahá