At the level of absolute truth, there is no reason to suffer. But at the relative level, we’re all in considerable pain. The cause of our discontent is our mistaken feeling of separateness. This isn’t based on anything tangible. It’s based on beliefs and concepts. The duality of subject and object, self and other, is an illusion imputed by the mind.
This absolute understanding is arrived at through the practice of letting go. Meanwhile, we can work at the level of everyday pain and treat other people’s suffering as our own.
From No Time to Lose: A Timely Guide to the Way of the Bodhisattva by Pema Chödrön, page 310