Sad Day, Week, Month, Lifetime

Im tying to figure out why some people are sad and others are not.  Is it genetic, is it attitude, is it low self-worth?  I have lived my life in sadness.  I think it stems from an early age where things happened and it just propelled all through my life.  But going back even further; I had a twin brother.  We shared a tiny space for 9 months.  We were best friends as well as twin siblings.  He was my everything.  Then we were born; first me, then him.  I was happy.  But something happened, I was left in the hospital and he went home.  Eventually, I came home too, but my twin brother, my best friend, my heartbeat, went back to the hospital and never left.  On a cold day in March, he went back to his heavenly home and I was abandoned.
I don't have any idea how my mother and father felt.  No idea if my mother grieved while taking care of me.  But I grieved too.  I am still grieving.  I had 2 brothers that passed away.  The older one was just a couple of hours old.  My twin was two months old.  What was God's plan.  The best of me when into that coffin.  My happiness left my body; don't know where that ended up.

So why are some people sad?  Why am I sad?  Why can't I just grab each day and bask in the possibility of what's to come?

My grandchild is suffering.  I have to feel her pain.  I feel everyone's pain. I feel sad.  Wouldn't you?