In the LeM of Rav Nahman of Breslov [and Uman] it is brought [vol II. perek 7] that R. Eliezer was on the level of son, while R.Yehoshua was on the level of servant. That is to say there is a tzadik who serves God on the level of son and another who serves God on the level of servant. And the level of son is איה מקום כבודו [like the question the serafim angels ask: "Where is the place of His Glory?"] The level of serving God like a servant is on the level of מלוא כל הארץ כבודו [that is the statement of the ophanim angels: "The whole world is full of his glory."]
And there Rav Nahman explains that even the tzadik who serves God on the level of son should still be aware of the level of "The whole world is full of his glory." And the tzadik that serves God on the level of servant should also be aware of the question "Where is the place of His Glory?" which shows when one reaches a higher level in the service of God, the more he knows how much more of a way there is to go.
I think this balance is a lesson for each person. Even as we gain in understanding in Torah, we ought to retain the knowledge of how far we really are. And yet not be so discouraged as to think progress in impossible.
