The speaker is describing a woman he admires, and the comparison he picks is vanilla. At first it sounds like a compliment: she is sweet and beautiful, so much so that ordinary sugar tastes bitter next to her. But the word "complacent" and the mention of her "monotony" show that something bothers him. She is lovely, yet she never pushes herself forward.
That tension is what the speaker calls his dilemma. He can't find words big enough for her beauty, but he also can't be honest about how dull she seems to him without piling on adjectives. His feelings are mixed. Her sweetness is a comfort, a "friend," and her beauty brings relief, but it also brings shame, perhaps because he feels guilty for wanting more from her. The closing question, asking why she can't be more like chocolate, is playful but a little wistful. The mood is affectionate, restless, and slightly awkward, like someone praising and criticizing in the same breath.