Why do we tend to have negative feelings?

Excerpt from Mr. Adnan Oktar's Live Conversation on A9TV dated February 18th, 2018

VTR: Why are our inner worlds always infested with negative emotions?

ADNAN OKTAR: Becoming perfect requires the presence of negativity. Positivity alone is not enough to achieve perfection, that is, to perfect the spirit and the mind. If everyone was decent and perfect, it would weaken our spiritual world. Resistance is required to become stronger. For example, the heavier the equipments one uses when working out, the stronger the body becomes. The same applies to the spirit as it becomes stronger in the face of adversity and abnormality; the spirit grows more mature, attentive and gracious, and keeps the person in balance. In the opposite case, people turn out to be blunt, impertinent and ungracious. Imagine heaven to be full of such impertinent people. Such people would never be able to enjoy the beauty and delights heaven offers; because to be able to do so, one first needs a normal person with rational mind. Therefore, people have to go through such training. There is no other way. Has anyone heard such a thing as 'fun' training? Any training is inherently arduous. Military trainings, for example, are always grueling.