Two lines, one question, and it works like a small trap. The speaker starts with a condition: suppose the little things, the passing details of a day, turn out not to matter once we look back on them. Then comes the quiet punch. If that's true, what makes us think the big things matter either?
The logic runs on an unspoken idea: big things are made of little things. A wedding is a thousand small moments, and so is a career or a friendship. If the small pieces fade or prove meaningless, the whole structure loses its footing. The title's "little things < big things," shown in the poem's heading, seems to set up the usual ranking, and the question then undermines it.
The speaker doesn't sound angry. The tone feels thoughtful, a little uneasy, like someone turning an idea over late at night. The mood is reflective and slightly unsettled, because the question is left open. The poem doesn't answer it, so you're invited to decide what you think matters, and why.