Antojar means 'to crave' or 'to feel like doing something on a whim'. It's a less common verb than 'querer' (to want) and carries a nuance of spontaneous desire or caprice.
In the song, the lyrics say, "De acariciar tu piel cada vez que se le antoje" (To caress your skin every time he feels like it). This highlights the casual and effortless access the other person has to the object of affection, emphasizing the singer's longing and envy for that freedom.
Have you ever been completely smitten with someone, but they're with someone else? That's exactly the feeling Regulo Caro captures in "El Lujo De Tenerte" [The Luxury Of Having You]. He's watching this person he adores, and he can't help but notice how lucky their partner is. He sings, "Qué suerte tiene él / De abrazarte por las noches / De acariciar tu piel / Cada vez que se le antoje" [What luck he has / To hug you at night / To caress your skin / Whenever he wants]. It's a mix of admiration and pure envy, seeing all the things he wishes he could do.
He really lays out how much he wants to be with this person, even admitting, "A ti nada te falta / Pero a mí me faltas tú" [You lack nothing / But I lack you]. It's a pretty blunt way to say he feels incomplete without them! He sees them as his "amor de mi vida" [love of my life], and the person who actually has their kisses? Well, he calls them "el idiota / Al más que le tengo envidia" [the idiot / The one I envy the most]. It's a relatable, if a little dramatic, look at unrequited love and longing for something that feels just out of reach, like the moon.