Matthew Poole and continuators on Genesis 21:11

11 And the thing was very grievous in Abrahams sight , because of his son .

Because of his tender affection to him, and Gods promise concerning him: See Gen. 17. 18, 20. He, who chearfully parted with Isaac was hardly brought to part with Ismael, because the former was done by Gods command, which he was obliged to obey; the latter by the passion of an enraged Woman, wherewith he thought not fit to comply; and probably he had denied her desire, if God had not interposed in it,

He doth not say because of his wife; from whence may be gathered either that Hagar was not properly his Wife; or that this was another of Abrahams infirmities, that he had not that affection for her which he should have had. Whereby we may also see the excellency of Gods institutions, who appointed but one Woman for one man, that each might have the intire interest in the others affections; and the danger of mens inventions, which brought Polygamy into the World, whereby a mans affections are divided into several, and sometimes contrary streams.