Let us enlightenment ourselves, What could be some causes of this?
Could it be stress, elaborate passwords or something else (please state what you think is a cause).
The only person I've seen in recent times that happens to lost his private key is actually a friend. I helped him bought few TRY about a year and half ago only for him to call me few weeks ago asking for how he can recover his wallet without private key. I think the major reason people usually forget their keys out of negligence is usually because the asset is small in value and maybe some other reasons too