Cosquillas literally means "tickles". It's a fun and common word, but Ivan Cornejo uses it in a very unique and poetic way in this song.
He sings about someone who gives him "cosquillas en la mente" (tickles in the mind). This beautiful metaphor describes the feeling of constantly thinking about someone in a happy, exciting way. It's as if the memory of them is a pleasant, persistent thought that you just can't shake.
If you have ever felt completely useless after a breakup, Ivan Cornejo gets it. In "Vuelve", he skips the poetic metaphors and goes straight for funny, messy honesty. He complains that his ex left him "como un traste sin lavar" [like an unwashed dish], and he is messing up constantly. In Mexico, making a mistake is called regarla (literally "to water it"), and Ivan admits "eso de regarla diario para mí es normal" [messing up daily is normal for me]. In fact, things are going so badly that he wishes he was never born, whining that he was "tan a gusto... allá en la panza de mi 'amá" [so comfortable back in my mom's belly] and "antes de poder caminar" [before being able to walk].
When he begs his ex to come back, the reasons get weirdly specific and sweet. He admits his mood crashes into "la depre" (short Spanish slang for depression) without her, and his heart races "como un perro al verte llegar" [like a dog seeing you arrive]. He even wonders if reality is fake, asking where his soul went "si esto es una matrix" [if this is a matrix]. He tries dating other people, but nobody compares—right down to the breakfast bowl, because "nadie me lo hace como tú el cereal" [nobody makes cereal for me like you do]. It is dramatic, self-pitying, and totally charming.