The origin of suffering is attachment to transient things and ignorance of the temporary nature of things. Transient things do not only include the physical objects that surround us, but also ideas, and, in a greater sense, all objects of our perception. Ignorance is the lack of understanding of how our mind is attached to impermanent things.
The reasons for suffering are desire, passion, ardor, pursuit of wealth and prestige, striving for fame and popularity, or in short: craving and clinging. Because the objects of our attachment are transient, their loss is inevitable, thus suffering will necessarily follow.
Objects of attachment also include the idea of a “self” which is a delusion, because there is no abiding self. What we call “self” is just an imagined entity, and we are merely a part of the ceaseless becoming of the universe.