It took Spike sitting down on Optimus's desk and asking for relationship advice to really drive home a certain fact for the leader of the Autobots. Since coming to Earth and adapting to their forms and culture, he had slowly gained an inkling of it. Spike was so human, though, and yet treated it as so natural to come and ask him for advice.
He spent a few cycles researching patriarchal values and roles, considering what he found against what so many of the Autobots seemed to need from him.
At least the word 'father' sounded better than 'robot Jesus'.