Demente translates to "crazy" or "insane." While it can be a strong word, in this context, it's used to express an intense, almost overwhelming feeling of being smitten.
The singer says, "Esta morrita me trae bien loco, demente," meaning "This girl drives me really crazy, insane." It's an engaging word because it vividly conveys the depth of his infatuation, making it more impactful than a simpler word like "enamorado" (in love).
This song is a super fun, romantic declaration from someone who is absolutely smitten! Our singer is telling a friend, "Viejo, esta vez no sé qué hacer" [Man, this time I don't know what to do] because this girl has him completely crazy, "bien loco, demente" [really crazy, demented]. He's so into her, he wants to steal a kiss and says she's totally different from his ex because this new girl wants something serious, and he's ready to give it to her.
He can't get over how beautiful she is, saying "estás re chula, de más" [you're super beautiful, too much] and that her "ojos hermosos" [beautiful eyes] would make anyone approach her. He's confident in their connection, stating "Me quiere y yo la quiero, yo la sabré cuidar" [She loves me and I love her, I'll know how to take care of her]. The chorus, "Y así lo quiso Dios, tenernos juntitos los dos" [And that's how God wanted it, for the two of us to be together], really drives home the idea that their love is meant to be. He's ready to make love "una y otra vez hasta que sí nos cansemos" [again and again until we get tired] and even jokes that if anyone looks at her funny, he'll pull out his "superón" (a slang term for a gun) – a bit much, but it shows how protective he feels! He also promises to call her "tarde o temprano" [sooner or later] to pick her up, and playfully notes, "Por el día somos santos, según aparentamos" [During the day we're saints, or so we pretend]. It's clear he's head over heels, declaring, "No te quiero, te amo" [I don't just like you, I love you].