It's a sad fact that, all too often, we don't appreciate our parents and everything they do for us. It's that attitude of unthinking ingratitude that forms the dominant theme of "Those Winter Sundays." The speaker's father wakes up bright and early on those ice cold Sunday mornings in winter to start a nice, warm fire. He even goes to the trouble of polishing the speaker's good shoes. Yet no one ever thanks him. This is a hard-working man, a dignified man who does his level best to provide for his family. But his efforts go unappreciated.
The poem's final question "what did I know, what did I know of love’s austere and lonely offices?" tells us that the speaker was simply too young to realize that his father acted out of love for his family. He took his father for granted, as most of do with our parents at some point, instead of thanking him for his loving kindness. Yet we remain sympathetic to the speaker. We forgive him for his innocence as we must also forgive ourselves for our similar youthful transgressions. And at least now, years later, the speaker has finally acknowledged his ingratitude and attained to a degree of wisdom and understanding, which is wholly admirable.