Love and Sacrifice
Our speaker grapples with the reality that, while falling in love has been both a natural and a rewarding experience, it has also opened him up to negative emotions that he otherwise would not have experienced. Essentially, he argues, love and heartache are two sides of the same coin. As he explains, neither he nor his lover would be feeling heartbroken if they'd simply never met, or never fallen in love. This doesn't mean that the speaker regrets his love. In fact, he says, "I'll ne'er blame my partial fancy." But this isn't necessarily because he thinks it's superior to love and suffer rather than to protect his feelings. Rather, as he states, "Naething could resist my Nancy," which is to say, there's no point wishing things had gone differently. Rather, his falling in love and subsequent heartbreak were almost inevitable.
Loneliness
While this is a love poem, it speaks more vividly about the speaker's own negative feelings than about the presence of his lover. This doesn't mean that he's self-centered—he displays a sincere devotion, wishes her well, and notes that she is as heartbroken as he. But his references to her are abstract, in contrast to the "heart-wrung tears" and "Dark despair" he describes himself as experiencing. Overwhelmed by his own sensations but tenuously in touch with the lover's, the speaker seems to be experiencing a kind of loneliness and isolation so intense that he cannot escape his own feelings or clearly see much of the outside world. The poem's repetitiveness—its tendency to rhyme words with themselves, or to repeat entire lines—yet again hints at extreme loneliness, showing words and phrases brushing up against themselves, just as the speaker can only seem to encounter himself at every turn.
Altruism
Though he is resigned to a lifetime of sadness himself, stressing the degree to which he has no hope of feeling better, the speaker enthusiastically wishes his lover a life of happiness, peace, joy, and various other lovely things. Rather than conveying bitterness, Burns shows how much his speaker wishes for his lover to have a good life without him. In fact, he implies, his hope that his lover will be happy is the only thing that saves the speaker from total, uninterrupted despair. Burns implies that his speaker's love causes him to prioritize the happiness of his loved one, even when that happiness excludes the speaker himself. Love and altruism, then, are inextricable from one another in this poem, and in the speaker's relationship.
