Why do humans generate complex emotions?



It all started with the primate ancestors of humans. As they evolved in tress, they evolved a more binocular vision to deal with the problem of hand-eye coordination to better their skills for swinging between branches. This created another problem.
Binocular vision is more suited to predators as :

1. They have to keep better track of their prey as their brain does better depth perception in forward facing eyes.
2. They don't have to look behind for any predators.

So, for preys, both side facing eyes are better suited as it gives them a better view of their predators.
Monkeys are not predators, yet evolved binocular vision.
To get around this problem they had to socialise and rely on other individuals of their species for survival. Their brains had to be receptive of the slightest emotion on the faces of their 'friends'. Their brains grew as they had to remember more faces now.
Humans just took it a little further from there