I have two siblings, and in our family the kids share almost everything, especially somewhat more expensive things such as movies and games (a Batman figurine belonged to an individual child, but the Playstation and all the games were public property).
We even shared a single save file for our first plystation game: Spyro the Dragon. It seemed so vast (we were between the ages of 7-11) that we believed a collaborative effort was needed to complete it. Separate save files on a single memory card eventually became the standard of the PS2 era.
Then a single event change everything. My little brother overwrote my FFX save file. HOURS of perfectionist, completionist work (sort of how i approach JRPGS) were simply gone. I was PISSED! I was caught between expressing how distraught i was and admitting it was a mistake.
And it was an honest mistake, 3 maybe 4 save files with the same character pictures side by side, the only difference being the location written in small text. This is when our parents decided that separate memory cards were needed. Now we each have a profile on our PS3.