Suspicion leads to distrust and makes relationships volatile. As a result people would be deprived of each other's good and pleasant company and this would weaken social ties.
The Qruan has forbidden us from being supicious of others because sometimes it would be a sin to suspect others,
O you who have faith! Avoid much suspicion. Indeed some suspicions are sins. (49:12)
That is because suspicion affects one’s thoughts and feelings and changes one’s behavior toward others. Suspicion leads to distrust and makes relationships volatile. As a result people would be deprived of each other's good and pleasant company and this would weaken social ties.
This is all when one suspects other human beings. Sometimes, one may suspect God, which is even more grave than suspecting other fellow human beings,
That He may punish the hypocrites, men and women, and the polytheists, men and women, who entertain a bad opinion of Allah. For them shall be an adverse turn of fortune: Allah is wrathful with them and He has cursed them, and prepared for them hell, and it is an evil destination. (48:6)
That supposition of yours which you conceived about your Lord has ruined you, so you have awakened among the losers. (41:23)
We have to be careful and not let problems, hardships, poverty, debts, illnesses and the like make us suspect God's mercy and might since this would mean distancing from God.