Small profit and Huge lost habit

I do not understand why Some traders are very uncomfortable to have a  position which is  in profit so  They cut thier profit very early, But They are so position on their  lost position   and let the lost get  bigger and bigger and They hope the market will  back .
The result of this mistake is That They May have 5 small to HUGE Profits and HUGE  lost That is bigger than the sum of all profit and Their day  end with Huge lost.
How to solve this problem is easy? Have a plan Before  open  any position, you should know When you should  close your position if the trend is favorable and never ever trade  without stop lost.