Albert Barnes Commentary Matthew 22:39

"And a second like [unto it] is this, Thou shalt love thy neighbor as thyself." — Matthew 22:39 (ASV)

The second is like unto it (Leviticus 19:18). It resembles it in importance, dignity, purity, and usefulness. This had not been asked by the lawyer, but Jesus took occasion to acquaint him with the substance of the whole law. For its meaning, see the notes on Matthew 19:19.

Compare Romans 13:9. Mark adds, there is no greater commandment than these. None respecting circumcision or sacrifice is greater. They are the fountain of all.