Jamás translates to "never" or "never before." While nunca is a common word for "never," jamás carries a stronger, more emphatic tone, often implying a sense of finality or an unprecedented event.
In "Dulce Amor," Selena sings, "Jamás había sentido lo que siento yo por ti" (Never before had I felt what I feel for you). This use of jamás powerfully conveys the unique and overwhelming nature of her love, emphasizing that this feeling is unlike anything she has experienced before. It's a word that adds significant emotional weight to the lyrics.
The first verse never says who. It describes the love instead, as a thing she owns and keeps somewhere safe: Yo, yo tengo un amor (I, I have a love), Este amor lo guardo en mi corazón (I keep this love in my heart). The second verse turns and speaks to him, and the verb moves with it, from tengo (I have) to eres (you are): Tú, tú eres este amor (You, you are this love), Y no puedo vivir sin ti (And I cannot live without you).
The chorus stops describing and starts declaring. Jamás había sentido lo que siento yo por ti (I had never felt what I feel for you) is a claim only somebody completely sure of themselves makes, and Solo vivo para ti (I live only for you) doubles down on it. Selena sings that four times with no variation, which is the point of a song built out of twelve distinct lines. Nothing is being worked out here; it was decided before the first line.