The poem sets up a quiet comparison. Caribou travel and rivers flow, and nobody asks either of them to explain themselves. They simply move, because moving is what they do. Then the speaker steps into the picture. When she goes to see the caribou or walk beside the river, other people want to know why.
That last line carries the poem's small sting. The speaker seems to long for the same ease the animals and the water have, the freedom to go toward something without defending it. Judging by the way the question is left hanging, "Others ask me why" feels slightly weary, as if the wish to be near wild things is not quite understood by people who expect practical answers.
The mood is calm, plain, and a little lonely. The short lines and simple wording match the speaker's point: some pulls are natural and don't need a justification. The poem quietly suggests that she belongs with the caribou and the river more than with the questioners.