Happiness is v short lived. Instead when issue creating tenson remains in memory for long. As we have been over told that to smile we need less energy compared to being sad. Then why it happens we become sad for long, think for issue more which seldom even needs attention.
Is it that our body is oriented towards negativity? Think about birth of new baby. After pain like breaking many bones mother gives baby a birth. With birth pain should go. But it goes slowly and throughout life this pain remains in memory. This pain is for birth but other pains too remain.