Gandalf's words to Denethor

[Artanis]

From LotR, Minas Tirith (my emphasis):

‘Well, my lord Steward, it is your task to keep some kingdom still against that event, which few now look to see. In that task you shall have all the aid that you are pleased to ask for. But I will say this: the rule of no realm is mine, neither of Gondor nor any other, great or small. But all worthy things that are in peril as the world now stands, those are my care. And for my part, I shall not wholly fail of my task, though Gondor should perish, if anything passes through this night that can still grow fair or bear fruit and flower again in days to come. For I also am a steward. Did you not know?’

This is Gandalf's words to Denethor, after the Steward has been questioning Pippin about Boromir's death.

"For I also am a steward." It is a remarkable statement. What do you think it means?