They still remain in the subconscious to a lesser or greater extent and exert their influences when triggered by current events. It is not necessary to remember all the events or use hypnosis to do this. It is not necessary to tell the therapist anything about the event itself, only the people involved. Easiest to alleviate is long-standing depression. If the cause is something that is over and done with for some period of time, like months or years, it's easier.
Depression is almost always the result of numerous upsetting incidents from the past.
In person or by phone, I take out the stresses one by one or grouped. I usually take out 8 or so for the average depressed person. It gives me a starting point. Acute depression is often helped but because the condition causing it may still exist, results are not as good. This may require professional help or even drugs.