Bad experiences and traumas
Psychological traumas are basically painful and difficult experiences. They make it harder to cope with life situations that remind you of them.
When someone as a child has been beaten, this can make him unnecessarily distrustful of people later in life.
A soldier who has been in dangerous war situations, can develop PTSD (Post traumatic stress disorder), where distressing life situations can make him overly aggressive, or afraid for his life.
Someone who has escaped a house that's on fire, could get very worried about burning candles, being afraid that they could top over, and start a fire.
To lessen the impact of such traumas on one's life, it works well for many people to process the emotions related to the traumatic experiences. Facing them helps to lessen their intensity. One can then more easily react aptly to life situations.
When being severely traumatized, like with PTSD or sexual abuse, life can be really difficult. It is better to get specialized treatment for that, rather than start with processing the emotions with my help.