John Calvin Commentary Genesis 18:3

"and said, My lord, if now I have found favor in thy sight, pass not away, I pray thee, from thy servant:" — Genesis 18:3 (ASV)

Pass not away, I pray thee, from thy servant. In asking so meekly, and even suppliantly, there is no doubt that Abraham does this, motivated by the reason I have stated. For if he had slaughtered calves for all kinds of travelers, his house would soon have been emptied by his lavish spending.

He, therefore, honored their virtue and their excellent qualities, so that he would not show contempt for God. Therefore, he was not so liberal as to invite wanderers, or other men of all kinds who flock together; nor did ambition lead him to deal so generously with these three persons, but rather his love and affection for those gifts of God, and those virtues which appeared in them.

Regarding his offering them merely a piece of bread, he downplays an act of kindness he was about to perform, not only to avoid all boasting, but also so that they might more easily consent to his counsel and his entreaties, once they were convinced that they would not be too burdensome or troublesome to him.

For modest people do not willingly cause others expense or trouble. The washing of feet, in that era and in that part of the world, was very common, perhaps because people traveled barefoot under burning suns: and washing feet parched by the heat was the primary remedy for relieving weariness.