many times while trading we let our goals down and get greedy or fearsome. the problem are the reason behind, mostly psychological reasons.
two scenarios

a. price went +20% up, we dont sell.
reasons
 we dont need the money now
 we anticipate more gains         (greed, market sentiment)
 we see others with more gains (greed, jealousy)
 we dream of million                 (greed, new life vision)

b. price went -25% down, we sell.
reasons
 we need the money now
 we are fearing it will crash more          (fear, market sentiment)
 we doubt our initial decision to buy      (judgement by single point of time)
 we see others leaving the investment   (fear, mass psychology, group)
 we see other investment that are not -25% (fear)


thats why many books advice to trade in smaller steps.  dont go with a gut feeling, set a goal and stick to it.

the biggest impacts to our psychology? maybe the media, the news, the rumors.
we read something and anticipate that it will move 'others' and so we act on our own.
we do it, but we have no idea what it should mean. what should a musk tweet mean to a price? 3%? 10%? what should india banning crypto mean?