To understand why, imagine a respectable father who shows kind interest in an orphaned youth and wishes to adopt him as a member of his own family.
أَفَلَيْسَ مَنْطِقِيًّا أَلَّا يَضُمَّهُ إِلَى عَائِلَتِهِ مَا لَمْ يَقْطَعْ لَهُ وَعْدًا رَسْمِيًّا بِذلِكَ ؟‏