The last look through an open door can carry more attachment than the goodbye spoken inside.

Goodbyes said face to face are usually managed. You keep your voice even, you say the sensible thing, you try not to make it harder for anyone. What you feel arrives a moment later, in the doorway, when nobody is watching and there is nothing left to manage. That last look holds the part the conversation was too careful to include.

If a parting is coming, it can help to say one true sentence while you are still in the room. And if you did not, the feeling in the doorway is not a failure; it is the same affection arriving on its own schedule.