Well, you only need the light when it's burning low,
Only miss the sun when it starts to snow,
Only know you love her when you let her go.

Only know you've been high when you're feeling low
Only hate the road when you're missing home
Only know you love her when you let her go...
And you let her go.

 Passenger - Let Her Go Lyrics | MetroLyrics 

People only regret when their most important person left. 

Kwon Ji Yong was caught red-handed making out with a girl by his girl friend. He trys to get her back but she is gone forever.