Pessimistic thoughts are only good to bring bad things... whereas optimistic thoughts give you energy and strength.
If you are sad, worried, disappointed... well, let the feelings go, feel them, but learn from that bad situation. We are humans! We cannot know everything... we are always learning.
We have to keep the good things, even in the worst situations...
So... it is a good quote: "What doesn't kill you, makes you stronger!"
And here you are, a Kelly Clarkson's good song... it says the same I am trying to explain. This is an optimistic song, about a broken relationship... but looking at how strong I am, even when you think I am weak.
What do YOU think?