I think it's a "glitch". Our brains are wired towards pattern recognitions, and this often manifests itself in recognizing seemingly random shapes as faces because of a vague resemblance.

It is my belief that music is a similar trait where we recognize patterns in rhythm, sound, and motifs as something pleasant because we are built to recognize and categorize so many things to the point where a series of sounds can give a pleasant feeling once we're able to "parse" it.

Source: None, really. Just me having given this a lot of thought in the past, as I'm very musically interested myself.