"O my Luve is like a red, red rose" is of the most famous poetic similes of all time and has survived into our modern American culture—hundreds of years later. The effect of this simile is positive. The speaker is actually addressing these lines to his "bonnie lass" and professes his undying love to her. Red roses are often symbolic of true love and romantic desire. The speaker compares his love to roses which have bloomed in June, which is a time of new beginnings. This simile therefore portrays the speaker's love as both true and "newly sprung," yet he swears that it is a true love.
The speaker goes on to compare his love to a melody that is "sweetly played in tune." A melody is a series of notes which, when played together, create a pleasing sound. This is often the focal point of a harmonized musical selection. This simile furthers the claim of the speaker's adoration for his "bonnie lass." It takes some talent to play a beautiful melody, and the speaker professes that his love is just as beautiful. His love is pleasing, worth listening to, and sweet.
These similes establish a tone of adoration from the very first lines; the speaker continues to build upon these professions of a true and beautiful love until the last lines, when he vows that he will love his "bonnie lass" until the end of time.