Sadness comes and goes as it pleases. It doesn't ask for permission to stay at your place, it simply invites itself in, grabs a few pillows and a blanket and forces itself into your heart for a slumber party.
It may be gone by morning. It may not.
More often than not, it's still there the morning after and gets up to make pancakes for both of you.
Yeah, Sadness is a childhood friend; one that followed you through trials and errors, always there. Sadness is the type of friend that'll stay 'til the end. For good or for worse.