Avraham at the opening of the tent

Why did Hashem appear to Avraham as he sat at the entrance of his tent? The Noam Elimelech explains that the Divine Presence rests on a person who considers himself small. Avraham, despite his greatness, sat at the doorway as one standing at the threshold, never assuming he had arrived. True humility is not a lack of self-worth; it is the open space that allows Hashem's presence to enter.