The mood in "To A Mouse" is one not only of sadness for what he has done to destroy the mouse's winter home while he was out ploughing.

He also apologizes that the world of human beings has encroached on her small existence, even in that she wants so little for herself;

I'm truly sorry Man's dominion 
Has broken Nature's social union,

he also can empathize with her:

...thy poor, earth-born companion, 
An' fellow-mortal!

As he continues to accept the blame for her present and dire circumstances of a home destroyed in the cold of winter, which might kill her...

Thou saw the fields laid bare an' wast, 
An' weary Winter comin fast, 
An' cozie here, beneath the blast, 
Thou thought to dwell, 
Till crash! the cruel coulter past 
Out thro' thy cell.

...he may also feel a kinship with her, inferred here, later in the poem.

That wee-bit heap o' leaves an' stibble, 
Has cost thee monie a weary nibble! 
Now thou's turn'd out, for a' thy trouble,

He believes that he, too, is in a terrible situation which he did not see coming. While he believes that her problems may be temporary, he looks back into his past and believes the consequences of that past may have far-reaching effects on his future, which to him seems very bleak.

Still, thou art blest, compar'd wi' me! 
The present only toucheth thee: 
But Och! I backward cast my e'e, 
On prospects drear! 
An' forward, tho' I canna see, 
I guess an' fear!

The primary emotions I detect from Burns' is sadness for the trouble he has caused this little mouse, the kinship he feels for their shared troubles, and his lack of hope for his own future.

(Burns is a well-known and well-loved Scottish poet whose work lingers long after his death.

His line

The best laid schemes o' Mice an' Men, 
Gang aft agley,

is often quoted, though translated:

"...the best laid schemes [plans] of mice and men / Go often askew [awry].

Though his dialect is often difficult to follow, he demonstrates his capacity to understand and identify with the feelings of others—in this poem, even with the lowly field mouse.

Expressing a sympathy with the mouse for his plight, Burns associates himself to a certain degree with the little creature of nature; at least, he feels some guilt:  "Ah, me, thy poor, earthborn companion."  For, it was while he was ploughing a field in order to earn some money that he disturbed the mouse's nest, an action that gave rise to his poem as Burns realized that has separted himself from "Nature's dominion."  So says enotes:

The warmth of Burns's verse arises from his humane attitude combined with the experience he had of being in close personal contact with the people about whom he wrote.

This "companionship" with the mouse causes Burns to reflect upon the similarities and differences that he and the mouse share.  Unlike the little mouse who has been turned out of her home and must scurry to find a shelter against the approaching winter, Burns will make money on the ploughing.  However, Burns likens himself to the mouse as they share the futility of making plans in a universe made cruel by the machinery of the Industrial Revolution:

But Mouse you and I are not alone

In proving foresight might be vain:

The best laid schemes of mice and men

Go often askew

And leaves us nothing but grief and pain

For promised joy!

But, contends Burns, the mouse is better off than man, for the mouse knows only the present, and does not worry about the future as the poet ruefully ponders:

But, oh, I backward cast my eye

On prospects dreary!

And forward, though I cannot see,

I guess and fear!