Theme: Love
Examples: Love the way you lie - Rihanna
Shape of you - Ed Sheeran
Stitches - Shawn Mendes
Secret love song - Little Mix
You don't know you're beautiful - 1D
...etc. The list is endless.
Why is the theme so popular?
1. People can relate to it
2. There are so many different aspects of it: Break ups, On and Off relationships, Turbulent relationships, Cheating...etc.
Hope this helps!