Why use simile?
Generally, a writer uses similes to enable the reader to imagine in his mind what the writer is saying. This is why a writer compares one thing to another with which the reader is familiar. The speaker compares his love to a red rose that has just bloomed. We are all familiar with a rose, so we are able to better understand the comparison. Most people are awed by the beauty of a "newly sprung" rose, so we can understand how the speaker feels about his love. He also compares to "the melodie/That's sweetly played in tune". Again, music is universal, so we can all understand how the speaker feels. We can imagine how we feel when we hear a beautiful piece of music played. It sends chills over us, and this lets us comprehend the speaker's feelings.
The initial two similes of the poem, and much of the rest of the speaker’s language, are commonplace. The speaker is obviously male, an energetic and fanciful man who does not mind exaggerating a bit in the interests of wooing his sweetheart and impressing his listeners. His situation is that he is declaring his love for his “bonnie lass” (line 5), vowing his love’s continuation, bidding her good bye, and promising to return. The charm of the poem lies in its evident sincerity and boldness, even if the language is unexceptional. This situation is like that in Donne’s “A Valediction: Forbidding Mourning”, except that Burns’s speaker does not exhibit the wit or sophistication of Donne’s speaker.