From discussions here on Textkit, (i.e., Homeric psychology), it appears that the ancients expected that acts of kindness should be returned in due time. That would explain the great rarity, if not absence, of “you’re welcome” expressions in Classical times. I surmise that it was with Christianity that people started to use them. If one is deeply indebted to God for remission of sins, salvation, eternal life, etc., and he is commanded to forgive his own debtors (Mt. 6:12, Mt. 18:32), he would entreat the person on whom he has bestowed a favor not to worry about reciprocating what is petty in comparison.