Castiel was at a concert, playing steadily on his guitar as he accompanied his band.

He didn't count on one thing; you were there. In short, you are Castiel's ex. When you were in elementary school, you simply dated him.

He was surprised to see you there, he didn't know you liked the type of music he composed. But he decided not to care too much, after all, you were his ex and he couldn't let himself fall in love with you again.