Yoga Sutra 1.4 tells us what happens when we do not calm the fluctuations of the mind.
When we are not still the mind does some crazy things. We form a habit of clinging to separateness: the ego identifying with mental activity (or stories). The whirl of our mental activity becomes captivating. This drama becomes so real that we lose ourselves in the theatrics and forget we are only witnesses to the play.
There are many examples of this in the external world now, but the simplest is the identification of all the roles we play in daily life (manager, mother, sister, husband, coach, care-taker, fat, skinny, etc). These roles may change frequently in the mind but they do not change our true nature, our inner Self.
When neutrality of the mind is lost the Self becomes the victim of the minds' exaggerations and projections. Identification with these fluctuations of the mind creates suffering
Sutra 1.5 starts to explain the different ways of suffering (staytuned )