This is a love song with a very specific image at its heart: when he left, he left behind a beso incompleto (an unfinished kiss), hoping that when he comes back he can "en tu boca encontrar su otra mitad" (find its other half on your lips). It is such a sweet, slightly corny idea, and Intocable sell it completely. Meanwhile the real situation is less romantic: she keeps hanging up on him, "siempre me cuelga sin hablar" (she always hangs up on me without talking). Colgar literally means to hang, and me cuelga is exactly the English "hangs up on me".
The chorus is a list of over-the-top promises. He would learn all over again how to make her fall in love, give her a thousand flowers, and "sobornaría a cupido" (I would bribe Cupid) before letting his heart walk away. Then comes the big one: "cada día será un 14 de febrero" (every day will be February 14th), Valentine's Day, which is el Día de San Valentín in Spanish. He ends with a dare, "Te reto a encontrar alguien que a ti te quiera como yo te quiero" (I dare you to find someone who loves you the way I love you). Notice the doubled a ti te quiera: Spanish repeats the pronoun for emphasis, like saying "loves YOU".