Much of our focus has been on the personal rewards of fasting. There is also great value in fasting on behalf of others. The fast that God applauds in Isaiah was the one where people gave their food to the hungry. 

A love for God is the best cause of fasting. The second best cause is when we do it as an act of quiet, unseen love for others.

Psalms 35:
11 Ruthless witnesses come forward;
    they question me on things I know nothing about.
12 They repay me evil for good
    and leave me like one bereaved.
13 Yet when they were ill, I put on sackcloth
    and humbled myself with fasting.
When my prayers returned to me unanswered,
14     I went about mourning
    as though for my friend or brother.
I bowed my head in grief
 as though weeping for my mother.

The Psalmist has people problems. While there is sometimes a rant asking God to retaliate against those who cause grief, here we see an attitude that fits the gospel. Jesus said that anyone could love their friends and family, but how do you show love for your enemies? This Psalmist chose to fast and pray for the ones who mistreated him.

In fasting he was able to discover a compassion that was comparable to the best friends and family relationships. When they became sick, he grieved for them.

With the Desert Fathers,

An old man was asked, 'How can I find God?' He said, 'In fasting, in watching, in labours, in devotion, and, above all, in discernment. I tell you, many have injured their bodies without discernment and have gone away from us having achieved nothing. Our mouths smell bad through fasting, we know the Scriptures by heart, we recite all the Psalms of David, but we have not that which God seeks: charity and humility.'[1]