As per Garud Puran and Hindu mythology, one of the reasons for suffering is the debts of your past birth. Your purpose of life is to face sufferings to pay these debts. The second reason is your present deeds till today starting from birth. If your sum total of bad deeds is more than good deeds, they get added to your previous birth’s debts.
The third reason for suffering is the form of struggle, which you entertain to attain future success. Some people do not call it as suffering.
The last reason for suffering is that some people acquire yogic powers to take on the sufferings of others. The classical examples are Shirdi Sai Baba and Jesus Christ who were known to cure others by adding their suffering to their own account. Most Gods or holy people had suffered in their last time, be it Jesus Christ, Krishna, Buddha or Sai Baba. Only Rishi Munis can remain alive and die at will even after they have paid for all their debts.