This life lesson comes courtesy of Fun. [link]
Some people will forget you in life, and move on without you (you will do the same to others). This doesn't mean that you are nothing, even if they don't think of you anymore.
There are a lot of other good life lessons in those song lyrics:
"Though I've never been through hell like that
I've closed enough windows to know you can never look back"
"If you're lost and alone
Or you're sinking like a stone
"May your past be the sound
Of your feet upon the ground"
And the big one:
"But I like to think I can cheat it all
To make up for the times I've been cheated on
And it's nice to know when I was left for dead
I was found and now I don't roam these streets
I am not the ghost you are to me."
There are more posters! (of ghosts, because ghosts are cool!)Pokemon Life Lessons Series
Pokemon copyright Nintendo/Gamefreak/The Pokemon Company International, apparently.