even after they have professed their love for each other.
Most Helpful Opinion
It wasn't love then, ill explain why.
Most people think they love someone when they find someone they have feelings for, Yes those feelings can develop into love but usually people have it mixed up with fascination or Lust or even both.
What causes them to break up? Life issues, they realized they weren't meant to be together, fights, conflicts, immaturity, lack of chemistry,etc, the list goes on and on. It just depends on what happened.