Desvelas comes from the verb desvelarse, which means 'to stay awake all night' or 'to lose sleep over' someone or something. It's a word packed with the emotion of longing and infatuation.
In this song's chorus, the singer dreams of being the person his friend thinks about so intensely that she can't sleep. He sings "Yo quisiera ser ese por quien te desvelas" (I wish I were the one you lose sleep over). This single, poetic word perfectly captures the feeling of lovesickness.
Have you ever been stuck in the friend zone? Reik's classic song "Yo Quisiera" is the ultimate anthem for that feeling. The singer is his friend's rock; he's her "mejor amigo" [best friend] and her "pañuelo de lagrimas" [handkerchief for tears] when she's crying over other guys. He gives her advice and comforts her, all while secretly wishing he was the one she was crying about!
It's a classic case of unrequited love. He wants so badly to be the reason for her strong emotions, saying, "Yo quisiera ser ese por quien / Te desvelas y te desesperas" [I would like to be the one for whom / you stay up all night and despair]. He's so afraid to tell her how he really feels, admitting, "Pero tengo miedo de que me rechazes" [But I'm afraid that you'll reject me]. He'd rather keep his feelings a secret than risk losing her friendship.