Those memories (especially painful or sad) are referred to as predominant because they carry overpowering effects, borne out of circumstances we mostly had no control over Some examples are the death of a loved one, terminal illnesses, accidents not caused by us, etc. When left unattended, our minds revisit the thoughts from these past events and re-enact them. We sort of experience them all over again and the overpowering feeling of helplessness returns. The moment that happens, we become incapacitated in the present by memories of the past and are unable to function anymore. As long as we continually dwell there, we remain mostly unproductive. Allowing our minds wander also causes us to wander.