When the narrator, while ploughing a field, destroys a mouse's nest, he speaks to the mouse as if it is a fellow human being. He states that he is the mouse's
poor, earth-born companion,
Most people don't think of mice as their companions, but the poet...
doesn't stop there. He continues to personify the mouse by speaking of its nest not as a nest but as a "housie," as if what he has destroyed is a human being's home: "Thy wee bit housie, too in ruin!"
He further personifies the mouse by attributing to it a human ability to plan and think ahead, rather than simply assuming that the mouse builds its nest out of instinct. You must, he says to the mouse, have been anticipating being all cosy and snug in your little house when the freezing days of winter come: "cozie here, beneath the blast, thou thought to dwell."
The identification continues in the famous line in which the poet says the best laid plans of mice and men can go awry, ("gang aft agley"), as if mice plan in the same way people do.
But then, in the last stanza, the poet backs away from the strong identification with the mouse that led him to personify it. Now, he thinks the mouse is better off because "the present only toucheth thee." In other words, it's as if he remembers suddenly that the mouse is not a human being and doesn't carry the burden of memory or worries about the distant future.