Humillen comes from the verb humillar, meaning "to humiliate." It's a strong and emotionally charged word that stands out in the lyrics, conveying a sense of deep disrespect and degradation.
In the context of this song, the singer expresses a dark wish for his former lover to be humiliated day by day and night by night until she cries, all in the hope that she will return to him. This intense and somewhat vindictive use of the word makes it particularly memorable and impactful.
Do not be fooled by how warmly this is sung. Almost the whole song is a list of curses aimed at an ex: may you never find a tender, honest love, may every man who turns up be an idiot, may they ignore you when you apologise, may you cry night after night with nobody there to comfort you. It is petty, theatrical, and recorded live with a crowd that is clearly enjoying every word of it.
The reason arrives in the title, para que regreses, so that you come back. Then the last wish gives him away completely: ojalá que nadie te ame como yo, half a curse and half a confession. Proud on the surface, wounded underneath, with Carin León's voice pushing it over the top. For a learner it is a fast tour of que plus the subjunctive, since almost every line uses it to wish something on somebody.