There are lots of discussions about this elsewhere on the net. Microsoft has been doing this for a while and I understand their reasoning --- the more people on a single operating system the easier it is for support. Tou would normally need to opt in to get the update and although they have sneaky ways to do this, its probably something you have agreed to, even if you didn't realize you were agreeing to it.

You can turn off the upgrade, you can say no to the upgrade. Next computer you buy will have it of course and that might be fine.