I was wondering if any of the early rabbis prior to Jesus might have hypothesized that God rejected Cain's sacrifice because Cain had not first reconciled with his brother Abel over some issue. This might explain the saying of Jesus in Matthew 5:23-24
"23 So when you are offering your gift at the altar, if you remember that your brother or sister has something against you, 24 leave your gift there before the altar and go; first be reconciled to your brother or sister, and then come and offer your gift." ( Bible Gateway passage: Matthew 5:23-24 - New Revised Standard Version )
"23 So when you are offering your gift at the altar, if you remember that your brother or sister has something against you, 24 leave your gift there before the altar and go; first be reconciled to your brother or sister, and then come and offer your gift." ( Bible Gateway passage: Matthew 5:23-24 - New Revised Standard Version )