why doesn't Safari recognize my password?

If the OP has the same issue as me, it is the following (which just started happening, my password is the same as always):

in Safari preferences a box pops up asking for my password when I want to make changes. In my case I want to see my saved passwords for various websites and make changes. I've done this before on this machine with the same password that I've had on it since day 1. Now though it doesn't work, the box just shakes and reappears. I haven't changed anything though I have done the software updates. I've had this computer (MacBook pro) for nearly 2 years.

So, like me, I think the OP is asking about the computer password no longer being accepted by Safari preferences. Of course my password continues to work to let me into my computer, it's just not working to make changes in Safari preferences.

Using Mac OSX - El Capitan & previous versions when we click on safari & its preferences > password we have to check the box of auto fill user name & passwords , so that they will be filled in the user library > keychain folder .

But after shifting to Mac OS Sierra , the settings has been changed for the security reason , when we click on password a big lock is shown stating : safari passwords are locked , to unlock the password enter the user name password .

Might be a corrupt .plist.

Do a backup, preferably 2 backups on 2 separate drives.

Quit the application.

Go to Finder and select your user/home folder. With that Finder window as the front window, either select Finder/View/Show View options or go command - J. When the View options opens, check ’Show Library Folder’. That should make your user library folder visible in your user/home folder. Select Library. Then go to Preferences/com.apple.Safari.plist. Move the .plist to your desktop.

Restart the computer, open the application, and test. If it works okay, delete the plist from the desktop.

If the application is the same, return the .plist to where you got it from, overwriting the newer one.

Thanks to leonie for some information contained in this.

With Safari open press Command , (comma) which will open Safari Preferences for you.

Select the Passwords tab.

Type in the web url for the site you're having problems with. If the site is listed, select it then click Remove.

Now open Keychain Access located in HD > Applications > Utilities

Select Password on the left

Type the website name in the search field.

If the website url is listed, right or control click that keychain then click Delete.

Back to Safari. Navigate to the site you want to that password stored for. Enter your username and password. Click Yes when prompted. That will create a new keychain for you for that site.