The speaker of the poem, who is presumably Wordsworth himself, assumes that this solitary Highland Lass is unhappy because of the song she is singing. The melancholy song seems strikingly appropriate to the setting, in which there would be complete silence except for the girl's song. In the first stanza he says:
Alone she cuts and binds the grain,
And sings a melancholy strain;
It seems only logical to assume that the girl is feeling melancholy if she is singing a melancholy strain. But further evidence that she might be feeling, let us say, blue is that she is all alone and that she is doing hard and tedious work which seems to have no ending, since she is only cutting little swaths of the grain with a sickle. Why is she doing this kind of work at all? Why is she doing it alone? Why isn't there some man to do the reaping? Or at least to be helping her with this all-important harvesting? It could be that she has lost a man--a father or a brother or a lover?
The speaker calls the song "plaintive." She is expressing pain.
Perhaps the plaintive numbers flow
For old, unhappy, far-off things,
And battles long ago:
Or is it some more humble lay,
Familiar matter of to-day?
Some natural sorrow, loss, or pain,
That has been, and may be again?
Even if she is thinking of "battles long ago," it may be because she has lost someone in a more recent battle among feuding Highlanders. But it seems more likely that she is singing about some "Familiar matter of to-day"? The poet would like to know her motivation for singing such a plaintive song and for singing it
As if her song could have no ending.
He sympathizes with her and would like to share in her sorrows. Wordsworth's definition of poetry has often been quoted:
Poetry is the spontaneous overflow of powerful feelings: it takes its origin from emotion recollected in tranquility.
No doubt the poet is writing about an emotion he had experienced a long time before, perhaps when he was on a walking tour. He is putting the scene in the past when he writes:
The music in my heart I bore,
Long after it was heard no more.