Wee, sleekit, cow'rin, tim'rous beastie, O, what a panic's in thy breastie! (1-2)
The speaker opens the poem by addressing the mouse, but he doesn't name the mouse right away. Instead, he opens with a list of adjectives to describe the mouse. And how does he describe it? He describes both its physical characteristics (it's small and sleek, or "wee and sleekit") and its emotional state: it's cowering and afraid ("cow'rin and tim'rous"), and it's in a panic. Right away, the speaker seems to sympathize with the mouse and to ask the reader to do the same. How can you not feel for a poor, shivering, frightened mouse?
I wad be laith to rin an' chase thee, Wi' murd'ring pattle! (5-6)
You know the stereotypical image of a person shrieking and chasing a mouse away with a broom? The speaker knows that stereotype, too, and he assures the mouse that he's not that kind of a guy. He says he would be sorry to chase the mouse with a "murdering pattle." (A pattle, by the way, is the handle of the plough.) It seems appropriate that the handle of the plough, which could represent human civilization and "man's dominion," as the speaker says later, should be used to threaten the mouse. So we know already in the first stanza of the poem that the speaker has a more sympathetic relationship to mice and to other animals than is typical.
I'm truly sorry man's dominion, Has broken nature's social union (7-8)
Whoa. Here, the speaker breaks out of his Scots dialect and includes two lines of standard English. The break in the dialect makes these lines stand out, so they must be important. The speaker apologizes on behalf of all mankind, and refers to the "social union" that mankind has broken with nature by taking control or "dominion" over the natural world. Sounds like it could be a reference to the theory of the social contract by philosophers like Rousseau and Locke in the eighteenth century. Basically, they argued that human society only functions because all the people in that society have a contract or agreement to act in certain ways (e.g., we don't run around pushing old ladies over or cutting in line). Even if these laws aren't written down, most people would agree to follow them for the sake of an orderly society. But the speaker of this poem says that there used to be a kind of social union or social harmony between humans and nature, but humans have broken it. Sorry, mouse.
The speaker claims that he has a lot in common with the mouse, after all—they're both poor, they're both inhabitants of the earth, and they're both mortal.
Till crash! the cruel coulter past Out thro' thy cell. (29-30)
The speaker personifies his plough when he calls it cruel. Again, the speaker emphasizes that the plough, a farming tool used to tame the wilderness and cultivate fields, is used to threaten or to hurt the mouse—and, by extension, all of nature.