Waking up on the right side of the bed

I've never thought that days were fair. Is it true what they say about waking up on the right side of the bed? One day, for no good reason, you can be so sad, and the next day so at peace. What is it about a day that changes these emotions inside of you? One day you can think of a sad memory you hadn't thought about in so long and it's a sad day. Then when a day is great, and you're so happy it's nothing more than a tease. No matter how many times you say you won't take the good days for granted, when they come along you always forget to appreciate that happiness. That happiness is gone with time, and a bad day will be worse in comparison. Then again, we wouldn't know happiness without sadness? It's a trap of emotions. It's not fair, and I'm confused by my own roller coaster of the two.