From Fee’s commentary on Philippians (NICNT):
“Without being maudlin or saccharine one may rightly note that Paul lived as a believer in a world surrounded by friends, that those friends brought him joy, and that the untimely death of such friends would have been for him immeasurable grief.”
It was, of course, a friendship centred on Christ – his way, his word, his kingdom, his forgiveness. If friends take delight in some common thing, then Christ is the common thing in which these friends took delight. Though this is not to deny the everyday, mundane delights in which they no doubt also shared.