John Lightfoot on John 19:26

[Woman, behold thy son!] I. "The widow is maintained by the goods of the heirs" [of him that is deceased] "so long as she remain a widow, till she receive her dower."

II. Joseph being deceased, and Jesus now dying, there were no heirs, and probably no goods or estate, for the support and maintenance of his mother Mary. This, Christ at his last breath takes particular care of; and probably had made provision before; for it is hardly conceivable that this was the first overture he had with St. John in this affair, but that he had obtained a promise from him, in his mother's behalf, some time before this. And hence perhaps that peculiar love he bore to him beyond all the rest. So that those words, Woman, behold thy son! and on the other side to him, Behold thy mother! seem no other than as if he had said, "This man, from the time that thou art now deprived of thy son, shall be in the stead of a son to thee, and shall cherish and provide for thee": and so, vice versa, to his disciple John.