This song really makes me cry so hard on the inside sometimes because the song sounds very lonely, helpless and very sad, and feels like the writer is talking to himself/herself the whole time.
It was sang by Karen Mok originally, released in 1999, to remember the day when a 7.3 magnitude earthquake struck Taiwan on 21 September 1999. The english translation for the title of the song is "Suddenly".
As the title said, the song talks about sudden twist in life, how you may be facing imminent death or on the brink of losing someone dear, then you realize and missed all the good things that had happened in your life with that important person. So important life lesson from this song is to cherish the people around you who are very important in your life.