Sobrar is a fascinating verb that means "to be left over" or "to be in excess". It's the opposite of faltar (to be lacking), which also appears in the song when he sings, "Pero me faltas tú" (But I'm missing you).
In the powerful line, "Me sobra juventud" (I have an excess of youth), Luis Miguel expresses that even something as precious as youth feels worthless and superfluous without the one he loves. It's a poetic way to say "What good is my youth if I don't have you?", perfectly capturing the song's theme of having everything but the one thing that truly matters.
This is a classic 'friend zone' anthem! Luis Miguel starts by admitting he was wrong about his life. He thought he was happy and had it all: friends, fun, and even some wild romances. But he realizes none of it matters because the one person he truly wants doesn't feel the same way. He sings, "Se ve que en realidad sólo me quieres / Como a un amigo más" (It's clear you really only love me / like just another friend). The whole song is built around this one painful truth.
The chorus is a huge, dramatic declaration: "Tengo todo excepto a ti" (I have everything except you). He feels his life is empty without this person. He even says that without their eyes, "Mis ojos ya no ven nada" (my eyes see nothing anymore). It's a song about that feeling when you have youth, success, and a full life, but it all feels worthless because the person you love isn't in it the way you want them to be.