Agonía translates directly to "agony" in English. It's a strong and evocative word that conveys intense suffering or distress.
In the context of "Llueve el Amor," Tito El Bambino uses agonía to describe the mental state of feeling like you've lost everything. He sings, "Pero es mental por simplemente darle paso a la agonía" (But it's mental for simply giving way to agony), suggesting that sometimes our suffering is self-imposed by dwelling on negative thoughts. This makes it a powerful word for expressing deep emotional struggle and the importance of overcoming it.
Tito El Bambino builds the whole song around one weather image and never lets go of it. "La lluvia ya no pasa por mi casa" (The rain no longer falls on my house) opens things on a drought, not a storm; the rose that used to brighten his morning "poco a poco se marchitó" (withered little by little). That's a strange way to open a love song, admitting the feeling had dried up, but it sets up the turn: Cupid aims his arrow again, and this time it lands.
The title pays off the moment the chorus hits. "Hoy llueve el amor" (Today love rains) flips the drought into a flood, and he doesn't ask for a season of it. "Que viva el amor de enero a diciembre" (long live love, from January to December) claims the whole calendar instead. It's a simple trick, love as weather, but he commits to it line after line rather than reaching for a second metaphor, and that restraint is what makes the chorus stick.