Serious question. If you have time, thanks.
Why nine mm?
1. IMHO, with modern expanding defensive ammo, a 9mm doesn’t give up much to a .40 or a .45.
2. A double-stack magazine in 9mm gives you 15+ rounds on hand. A double-stack mag in .45 makes for a very thick and bulky pistol. My Springfield Xd holds plenty of rounds, but is still small enough to conceal.
3. 9mm ammo is just about the most available of the common calibers. Especially when compared to .40 caliber.
4. Women and small adults with little training can handle a 9mm where a .45 would intimidate them with recoil, depending on some factors. (The worst for recoil will be compact .45s.) The entire family can use the same pistol, instead of the big scary one being “Dad’s gun” alone.
http://www.greent.com/40Page/ammo/9/9mm-advoc.htm
An older article advocating for the 9mm, but stands up to the test of time, IMHO...
Have a look...