The main reason is :- For Others.
Sufferings isn’t God’s boon on mankind or anyone. Mostly the suffering which we all have, is for others.
It starts from sacrifice, leading to extensive use by others.
Sufferings is many times for our own foolishness or silly askings, but that can be contained with self -discipline and understanding.
Suppose a friend asks for a ball from you, and you lend it to him. He doesn’t return it back despite you asking for it many a times. Many will say it’s just a ball, what does it matter? But for you that ball holds a sentimental value, as you had won that ball in a competition or it may have given to you by your mother on your birthday.
So intentionally your friend has caused you suffering.
Suffering is caused by the vices such as greed, love, fiendishness and stupidity.
One has to be very much careful that they are not taken advantage of by others.