He saying basically this girl is nothing special, she’s just like any other girl , yet he loves her all the same , and he guarantees that no one has ever loved her as he has.
"The three quatrains satirize common poetic comparisons of one’s beloved to beautiful things, suggesting that the speaker’s feelings are not strong. However, the sudden reversal in tone in the final couplet surprises and moves through its sincerity and depth of feeling, suggesting strong emotions.
