Luto literally translates to "mourning," the period of grief and sorrow after a loss. It's a powerful word that immediately sets a dramatic and melancholic tone.
In the song, Aventura uses this word metaphorically in the iconic opening line: "Mi corazoncito está de luto por tu amor" (My little heart is in mourning for your love). By saying his heart is in mourning, he elevates his heartbreak to a profound sense of loss, as if his unrequited love has literally died.
This song is a hilariously dramatic take on unrequited love. The singer, Romeo Santos, isn't just sad—his corazoncito (little heart) is literally in mourning for a love it can't have. He imagines it lighting candles and writing songs for this person: "Mi corazoncito está de luto por tu amor / Prendió dos velitas y te escribió una canción" (My little heart is in mourning for your love / It lit two little candles and wrote you a song). He even talks about his heart as if it's a stubborn child who thinks he owns this person, which he admits is pure imagination.
The whole thing is a secret obsession. He begs the object of his affection, "No le digas a nadie / Lo mucho que te quiero" (Don't tell anyone / How much I love you). He knows this love only exists in his head—he says it himself: "¿Qué importa que en mi mente sea tu hombre? / Tú no me correspondes" (What does it matter that in my mind I'm your man? / You don't feel the same way). But he doesn't care! He just wants to be left alone to fantasize, repeating the plea "Déjenme soñar" (Let me dream) because, after all, his heart is "mío, mío, mío" (mine, mine, mine).