In a longitudinal study, 66 percent of respondents were sad at first after the loss of a loved one but were happy and productive six months later. this demonstrates

The answer is Mourning.

Mourning usually means to express deep sorrow to someone who has died.
It is usually common to mourn especially when a love one dies. People often tend to cry a lot that they can hardly breathe and also most of us wears black clothes when we mourn. It is normal for people like us to feel the mourn because when we mourn we actually express those thoughts and feelings and to mourn is to be an active participant in our grief journeys. But, after mourning, life goes on and we go back to being ourselves and continue our daily routine in our own lives. 

It's considered to be association or connection