The Vocal-Master once told me that the more popular and catchier a melody is, the more repetitions one can find in it. Take Memory or My Way or Dancing Queen as an example: almost every phrase is a repetition of one of the previous phrases with some variations. Similarity helps memorizing the song, variation helps not to get bored.
In fact, all the melodies I can think of contain repetition and variation in every phrase. But what strikes me is that it resembles the natural language information structure very much. Repetition is a kind of Topic (old, given, assumed information), and variation resembles Focus (new, highlighted parts of the sentence). A difference is that the music hasn't any devices to express reference except for repetition, while sentence Topic may not be seen in the discourse before. It may be an indirect or anaphoric reference (A phone rang. A nice female voice asked who she's talking to), or a real-world entity that 'everyone knows' (The president visited England). But I still see a great deal similarity. Both music and discourse are developing in time, introducing new themes and information. What was new in one sentence, becomes given in another one; we can repeat what once was a variation.
The music theory, of course, knows a lot of various ways of developing melody. Music even has phrases and sentences, questions and exclamations. Moreover, music and language are processed by the same brain systems and there are theories of syntactic processing in music. It may even have semantics! Seems that it's only me who didn't suspect this language-music relation until recently.