“Or what man is there of you, whom if his son ask bread, will he give him a stone?”
Or which one of you, if his son asks him for bread, will give him a stone? The parental analogy that grounds the ask-seek-knock promise: no good father gives his hungry child a stone when bread is requested. The analogy moves from human parenting to divine parenting in two steps: even human fathers give good gifts; how much more does the heavenly Father? The reductio ad absurdum of the stone-for-bread image communicates the absurdity of doubting the Father's responsive care. The father who gives a stone to a hungry child is a monstrous parody of fatherhood; the Father who does not respond to his children's petitions is equally unthinkable given his character.
Church Fathers on Matthew 7:9
As above He had cited the birds of the air and the lilies of the field, that our hopes may rise from the less to the greater; so also does He in this place, when He says, 'Or what man among you?'
Lest perchance any one considering how great is the difference between God and man, and weighing his own sins should despair of obtaining, and so never take in hand to ask; therefore He proposes a comparison of the relation between father and son; that should we despair because of our sins, we may hope because of God’s fatherly goodness.
There are two things behoveful for one that prays; that he ask earnestly; and that he ask such things as he ought to ask. And those are spiritual things; as Solomon, because he asked such things as were right, received speedily.