In "To A Mouse," Burns argues that man has dominated animals and, more importantly, that this domination is wrong. This is clearly shown in the first and second stanzas, when the speaker apologizes to the mouse for breaking "nature's social union." In the speaker's opinion, the mouse is frightened of him not because of his stature or the shock of his arrival, but because of this domination. Had man not dominated animals, the mouse would show no fear towards the speaker.
Burns's tone is, therefore, apologetic and regretful in dealing with the topic of man's domination over animals.
In the seventh stanza, Burns argues that carefully laid plans often go wrong, as we see in the following line:
The best-laid schemes o' mice an' men
Gang aft a-gley.
In other words, no matter how carefully we plan, it often happens that something gets in the way. This disruption to our plans generally leads to pain and upset instead of the "joy" that we had hoped for.