Echar en cara is a powerful idiomatic expression that literally translates to "to throw in the face". It means to reproach someone or to constantly bring up past favors or mistakes in an accusatory way.
In the song, Alejandro Sanz sings, "Lo echabas en cara y luego te reías" (You threw it in my face and then you laughed). This line vividly paints a picture of a toxic relationship, where his partner would hold things over his head, making it a memorable and useful phrase for expressing frustration.
This song is all about giving yourself permission to have a bad day. Alejandro Sanz starts by saying if you see him crying, it's not because he's unhappy or in love, but because "a veces uno sólo llora porque sí" (sometimes one just cries for no reason). He wants to make it clear he's not sad about his ex, who was such a liar that when she said "'buenos días'" (good morning), he "tenía que salir pa' ver si era verdad" (had to go outside to see if it was true). That's a pretty funny way to say you can't trust someone!
The main point is that it's okay to feel down. He repeats the line "Sentirse mal también está bien" (Feeling bad is also okay) because bad feelings don't last forever. And he adds a sharp little insult to his ex: "porque no hay mal que dure pa' siempre, y menos tú" (because there's no bad thing that lasts forever, and even less so, you). He insists he can't miss her because she gave him nothing. It's a perfect song for when you're just in a mood and don't want to talk to anyone, and you need a reminder that the feeling will pass.