Windows 10 has a new feature called Wi-Fi Sense that makes sharing your Wi-Fi password slightly more convenient -- at the expense of network security, naturally.
Wi-Fi Sense automatically shares password information with your contacts, in theory so your friends and family will be able to connect to your network without having to manually enter in a password. Your friends won't actually know your exact password (it's encrypted and then stored in the cloud), but assuming you don't necessarily want every contact you have in Outlook, Skype and Facebook to be able to connect to your home network when they're in range of it, this is not an ideal setup.
Also, if you chose Express Settings when you installed Windows 10, Wi-Fi Sense is turned on by default.
So the first thing you'll want to do is turn this off. To do this, open up Settings and go to Network & Internet > Wi-Fi > Manage Wi-Fi settings and uncheck the options to share your networks with Outlook.com contacts, Skype contacts, and Facebook friends. Here, you can also turn off the ability to automatically connect to open hotspots and to connect to networks shared by your contacts.
But turning off Wi-Fi sense on your computer is only one (small) part of the equation. Because anyone else who connects to your Wi-Fi network with their Windows 10 laptop or phone is putting your network's security at risk if they haven't turned off Wi-Fi Sense on their device.
So the next thing you'll need to do is to make your network unshareable. To do this, you will need to open up your router's settings and change the name of your home network to something with _optout in it. The easiest way to do this is to simply add _optout to the end of your network -- for example, if your network is called SarahsWiFi, you will want to change the name to SarahsWiFi_optout. Adding _optout to your network name allows you to opt out of Wi-Fi Sense completely -- nobody will be able to share your network information with their Facebook buddies, even if they connect to your network.