When you are very small you see no reason for there to be locks or even doors on bathrooms. Not only that, but this room makes you smile. The bathroom is a fun room. This belief is held for a very long time. I'm told that bathroom privacy comes around the time they start school. My oldest now prefers privacy for herself, yet still prefers the open door policy for everyone else...in case she needs something...a juice refill, how to spell "mustard", or to monitor her brother's attempt to put pee-pee in the potty.
So later, privacy is a must. If you are in the bathroom for ANY reason at all, the door is locked. This continues for years, sometimes decades...all through the school years, the roommate years. Even when you get your own apartment, it is a habit to close the bathroom door, when no one else is home.
Then, you get married. You relax. You realize you married a smarty who understands when the door is closed, someone is in there. There is no need to lock it. If there is an emergency...a tornado is approaching, they needs to floss, Mr. Smarty can still get in, but privacy is respected.
Then you are married a little longer. You relax. Privacy is still respected, but so is time. If two can get ready for the day simultaneously, so be it. You welcome this little step in the name of Efficiency.
Then you are married a little longer. You have a child, then a second, and sometimes, a third. Now, an audience in the bathroom is routine. Although no longer is this efficient. In classic Chandler-style: This is so the opposite of efficient. But what's a mom to do? Lock her babies out? And listen to them beat on the door for the (gasp!) thirty seconds you left them unsupervised? Ummm, Yes. I think this the calm IN the storm. The half a minute of peace that a 2 inch faux wood door grants while on just the other side, a storm is raging.
Anyway, back to the circle of life. Yes, that is the title of this post. I will get there. At the time of parenthood with small children, bathroom privacy is desired again. This continues for a while. I cannot comment on the specifics of the duration because I have not experienced life beyond this season, yet.
Assuming I survive the bathroom entourage years, and live to a ripe old, old, old age, I imagine, this preference for privacy will decline yet again. I like to think that when I am a centenarian, I will have mastered all my issues of pride , and will welcome any bathroom company with a smile. (Ahhh, the cycle! I get it!)