The phrase "I want to tell you a secret" adds an element of intimacy and personal connection. It implies that the speaker is sharing something deeply personal and meaningful with the person they're addressing.
The assertion that "there is nothing more beautiful than you in this world" is a profound compliment. It signifies that, in the eyes of the speaker, the person is not only physically beautiful but possesses a unique and unparalleled beauty that surpasses all other worldly beauty.
In essence, this statement is a poetic and sincere expression of admiration. It conveys the idea that the person's beauty is exceptional and incomparable, making them stand out as the most beautiful presence in the speaker's world. It celebrates the uniqueness and radiance of the person they hold in high regard.