Sforno on Genesis 21:6/ספורנו על בראשית כ״א:ו׳

1צחוק עשה לי אלקים, even though my son has to endure the pain of the circumcision at such as tender age, nonetheless my heart is full of joy. Therefore, everyone who hears about this event will rejoice on my behalf ignoring the aspect of the circumcision.