Throughout the first several stanzas of poem "The Old Familiar Faces," the speaker recalls his childhood pals, his drinking buddies from later in life, a woman whom he loved, and a very kind friend.
He muses on all of these "familiar faces" and sadly says that they're all gone now, and he's left wandering around, hoping that he'll somehow find those familiar faces.
We don't know why the childhood friends or the drinking buddies are gone now, though we might guess that they simply drifted apart as the speaker got older, and we infer that the woman he loved broke up with him ("Closed are her doors to me"). We do know for certain that the speaker himself abandoned his close friend.
Now the speaker starts to focus on that one kind friend, sadly wondering why they couldn't have been brothers.
The poem ends as the speaker summarizes his depressing situation:
"How some they have died, and some they have left me,
And some are taken from me; all are departed;
All, all are gone, the old familiar faces."
The central idea of the poem, then, is that the people whom the speaker knew and loved and connected with are all gone now. Phrased differently, you could say that the central idea is an intensely felt loneliness and loss of companionship, or a lament for the loss of the people whom the speaker loved.