The thing that makes a melody memorable is the repetition of the motif or musical phase. The motif is a small musical phase that is used throughout a melody. By repeating the main motif it “embeds” the melody into the listeners ear. Think of it this way. If you want to learn how to do something real well. You will practice what ever moves or actions required to master that skill. In effect its the repetition that build a strong memory on how to perform it. The same is with a melody. By repeating a motif of phase several times will help the listener better remember the melody.