Albert Barnes Commentary Luke 17:1-2

"And he said unto his disciples, It is impossible but that occasions of stumbling should come; but woe unto him, through whom they come! It were well for him if a millstone were hanged about his neck, and he were thrown into the sea, rather than that he should cause one of these little ones to stumble." — Luke 17:1-2 (ASV)

It is impossible. It must happen. Such is the state of things that it will be. See these verses explained in See Barnes, Matthew 18:6.

See Barnes, Matthew 18:7.