I've instructed in PA 38's quite a bit
well...first off, always check the doors YOURSELF. Always push on the door and make sure the latches are engaged properly. Never trust a passenger to do anything right. Point out to the passenger how you open and close the door and how you are pushing on it to make sure it is shut.
and when in doubt...just fly wiith it slightly ajar, return to the field and fix it..and then wonder if you forgot anything else.
I still remember my PPL checkride...the examiner (one of the best pilots I've ever flown with) didn't engage the top latch on the PA 28 cherokee. Since I had learned to always push on the door before engine start I looked at him and smiled and shut the door. And if it is really too hot and you want the air flow, well check it when you do your before takeoff check and do it all the time. It isn't that hard to do or check.
look, things happen...seat belt gets left outside the door and it slaps the fuselage/door in flight...what do you do...land somewhere and fix it.
stuff happens...FLY THE FRICKING PLANE FIRST...always check everybodies seat belt/harness...check the cowling, th eoil cap everything.
an airplane is NOT an air going sort of car, its an airplane...treat it with respect.
and never let a passenger on or off the plane without the engine shut down, mixture and mags off...
you were flying over the ocean? do you carry life vests, over water devices? if you don'tk, you have to be gliding distance to the shore where I live.