Because he is sensible that he fares well with thee. Or, because it is good, i.e. acceptable in his eyes, or pleasing to him, to be with thee.
Related Commentaries of Deuteronomy 15:16
Deuteronomy 15:16
16 And it shall be, if he say unto thee, I will not go away from thee; because he loveth thee and thine house, because he is well with thee;