There is the possibility of post surgical depression, which happens with any long length surgery, than there are the case's where someone had surgery who didn't /shouldn't have had it, and than have post surgical regret.
IMO the individuals who are telling you this more than likely are them themselves uncertain, and using this answer as a crutch to cover a problem which they have yet to resolve.
I can tell you something about post-surgical depression as I was affected by it, as well as poor judgment to stop taking my anti-depressant a little to fast/too soon, but as far as surgery is concerned it saved my life.