"To Celia" is a mini love song of sorts. The speaker talks about how he doesn't need a real drink, only a cup that has been kissed by the woman he loves. (It sort of reminds us of that song from Harry Potter: "Oh, come and stir my cauldron, / And if you do it right, / I'll boil you up some hot, strong love / To keep you warm tonight." Or maybe that's just us.) Anyway… The speaker thinks that Celia's so angelic or special that she can, potentially, keep a wreath of flowers from withering. If "To Celia" is about love, however, it is also about how, sometimes, the things we love can also let us down, if only a little bit. After all, Celia does return the speakers wreath.
The speaker describes courtship and love as a type of drinking. His description of love is slightly less idealized by his resort to a metaphor that has a lot to do with taverns and bodily necessity.
While we feel that the speaker really loves Celia, it is also strange that he sends her a "rosy wreath" (9), not to honor her, but to see if she can keep it alive. We feel, if only for a moment, that his love is also partly some type of experiment.