How does pessimism distort our worldview?
The selfish man’s worldview revolves around himself. The degree of concern he shows to others is proportional to the degree they are connected to him. Among living beings he cares only for humans; among humans he cares only for his own family; and among his family he cares only for himself. He imagines himself to be at the center of the universe and his self-centered worldview, coupled with pessimism, results in a very distorted perspective of life.
Everything in this life is inherently beautiful. Almost everything is visibly beautiful to everybody. In exceptional cases the beauty may be hidden due seemingly negative reasons. However it is not possible for these to be acts of cruelty by God (astaghfirullah). Indeed, God has the power to cause immense suffering every second if He wishes to do so. Yet He wishes mercy on us.
Suppose for instance that you lose a loved one. As difficult as this may seem, it is crucial to observe the whole picture. The fact that you have other loved ones whom you did not lose, the fact that you are healthy, the fact that you have food and water and are able to breathe effortlessly are all signs of mercy. Thus, the visible mercy of God far outweighs the suffering. So much so that if we started counting all the blessings of God bestowed upon us we would not be able to finish doing so. The suffering is thus infinitesimally small compared to the visible mercy of God at any given time. It is therefore unreasonable to believe these acts are out of cruelty; there must be other purposes behind these calamities. The unhappy man, due to his pessimism, is unable to see the infinitesimal size of these calamities. This gives him a very distorted picture of the reality of life. It prevents him from realizing that this world has a Most Merciful Ruler.