Pasión translates directly to "passion." While it might seem like a common word, its appearance in this song is particularly evocative and central to the theme.
Julio Iglesias sings about wanting to "volver a vivir la pasión y el calor de otro amor" (to live again the passion and warmth of another love). This highlights a deep longing for intense emotion and connection, making it a powerful and enticing word that captures the romantic essence of the song.
The singer is looking for someone who isn't there yet. The whole song circles one question, "Quién será la que me quiera a mí" (Who will be the one to love me?), and answers it only with "Yo no sé, yo no sé" (I don't know, I don't know). He doesn't even know if he will find her: "Yo no sé si la podré encontrar" (I don't know if I'll be able to find her), or whether he can love again at all: "Yo no sé si volveré a querer" (I don't know if I'll love again).
The verse tells you why he's asking. "He querido volver a vivir la pasión y el calor de otro amor" (I've wanted to live again the passion and the warmth of another love): he has loved before and wants that feeling back. In this duet the mambo rhythm keeps the longing light, and near the end Thalía turns the question back on him, "Quién será la que te quiera a ti" (Who will be the one to love you), so the search starts to sound like a flirtation between the two singers.