From Alexander MacLaren's commentary on Genesis:
"The first great truth enshrined in this part of the story is that the friend of God is compassionate even of the sinful and degraded. Abraham did not intercede for Lot, but for the sinners in Sodom. He had perilled his life in warfare for them; he now pleads with God for them. Where had he learned this brave pity? Where but from the God with whom he lived by faith?"