Here are a few reasons. There are many more that are even worse.
http://www.extremetech.com/extreme/213979-microsoft-thinks-youll-love-windows-10-so-much-it-downloads-it-for-you-without-asking
http://www.forbes.com/sites/gordonkelly/2015/11/10/microsoft-admits-windows-10-automatic-installs/#26abdac505bf
http://siliconangle.com/blog/2016/05/18/microsoft-makes-one-last-extremely-deceptive-windows-10-upgrade-push/
http://arstechnica.com/information-technology/2015/08/even-when-told-not-to-windows-10-just-cant-stop-talking-to-microsoft/
http://arstechnica.com/information-technology/2015/08/microsoft-accused-of-adding-spy-features-to-windows-7-8/
http://thehackernews.com/2015/08/windows-spying-on-you.html