It is what it is, life is a suffering and the more you think about it the more you suffer. I've buried 5 dogs during my life and now can't have dogs because when i see a puppy with my eyes i see it dying with my mind's eye. It wasn't me who started all of this, I'm just forced to play this game by the rules that I didn't set.

So, from a happiness standpoint human being should not think much, especially about death, suffering of all kinds, diseases, etc. Plus, thinking much about this can impact socializing badly, nobody wants to be friends with a person who is always in sorrow.