Why do songs stick in our heads?

I read this article on BBC.

“Earworms” It means songs that get stuck in our brains and go round round…!According to the research it shows our minds or stresses. Sometimes earworms make us unpleasant, and finally one psychologist found solutions.

I guess that many people experienced this “earworms” phenomenon. If these songs are not favorite or unlike songs for you, these would be annoyed us. In my case, Kyari pamyu-pamyu’s songs are often repeated in my head. There are a lot of repetitions. For example,♪ “fashion monster” or “tukema” in her lyrics. It makes her songs catchy. Actually these are not irritate me.
Moreover, in the way of learning English class, I learned that “sing a song” is a one of strategy to learn English effectively therefore songs are very helpful to study so I want to use it for next vocabulary test.
Questions
1.Tell me, what or whose songs are often stuck in your head?
2.Have you ever used songs for study?