Antojar means "to crave" or "to fancy something." It's a delightful verb that expresses a strong, often sudden, desire for something specific.
In this romantic song, Christian Nodal sings, "Si solamente le quedara un beso a mi boca, Lo gastaría contigo, porque otra no se me antoja." He uses antojar to emphasize that he has no desire or craving for anyone else's kiss, highlighting his exclusive and deep affection for his beloved.
Nodal makes his case in small, concrete trades. If one kiss were all he had left, "Lo gastaría contigo, porque otra no se me antoja" ("I'd spend it on you, because I don't fancy anyone else"). If he had to pick one person for the rest of his life, the role is already filled: "Serías tú la que con el puesto se quedaría" ("you'd be the one who'd keep the position"). Puesto is the ordinary word for a job — a post, a vacancy — and putting it in a love song is both the joke and the promise. The same practicality runs through "Si voy a tener que pasar por buenas y por malas" ("If I'm going to have to go through good times and bad"): he assumes the bad ones are coming, and files them under things he can handle holding her hand.
The title turns on a distinction Spanish draws in three words. He has been in love before, "pero muy poquito" ("but only a very little"), and then the line the whole song leans on: "Porque, amor, lo que se dice amor lo conocí contigo" ("Because, love, what you'd actually call love — I met that with you"). Lo que se dice is the phrase you reach for when you want to separate the word from the real thing, the way English says properly or actually. He is not claiming he never loved anyone. He is saying nothing before this had earned the name.