Imagino que quieres mostrarle la carta a tu novio para que vea que no tiene por qué estar celoso.I imagine you want to show the letter to your boyfriend so that he sees there's no reason for him to be jealous.
Sé que quieres mostrarle la carta a Juan, pero prefiero que no lo hagas.I know you want to show Juan the letter, but I prefer you not to do it.