Asomar means 'to peek out' or 'to show up'. While it can be used literally, it often carries a more nuanced meaning of something appearing subtly or after being hidden.
In this song, Christian Nodal sings, "Contigo mi sonrisa se volvió a asomar" (With you, my smile reappeared/peeked out again). He uses asomar to convey that his smile, which had been absent or hidden, has now subtly and beautifully returned because of his new love. It's a tender and evocative way to describe rediscovering joy.
Nodal spends this one adding things up. He opens on the small stuff, Me gusta mirar tus ojos de cerquita (I like looking at your eyes up close), then Por el lado que te vea, estás bonita (whichever side I look at you from, you're pretty), and by the third line he has stopped arguing with himself: ¿Y cómo no? (And how could I not?). What tips it over is the accounting rather than her face. Si vienes incluida hasta con buen corazón (if you come thrown in with a good heart on top) is the language of a bargain, as if she arrived with an extra feature nobody charged him for, and Me hace decir que ya gané en la vida (it makes me say I've already won at life) is him closing the ledger.
The title carries the whole idea. Mi antes y después de ti (my before and after you) / No tiene comparación (doesn't compare) is a man re-dating his own life from the day she turned up. The best line is the confession sitting under it, Y según yo decía que sabía amar (and I used to go around saying I knew how to love), which he only admits was nonsense after Me presentaste a la felicidad (you introduced me to happiness). A small, unshowy song from Bandera Blanca, and it lands because he sounds slightly embarrassed about how much he means it.