"A Red, Red Rose" is one of the most famous love poems in the English language, and it's pretty clear why—the entire poem is an expression of the speaker's powerful, undying love. Line after line, he lets us know his love is lasting, for real, and awesomely awesome. So awesome, in fact, that he would even walk ten thousand miles to see his bonnie lass again.
Um, hello? Roses have thorns. Sure, it's nice and pretty, but it stings, too, and our speaker shouldn't forget that.
We might believe that love can last forever, but by comparing his love to a flower the speaker suggests that it, like the flower, may die someday.