Something causes Windows defender to stop working

Do you have another Anti Virus running as well as Defender? If you do not, I would suggest the following:

You may wish to consider installing Microsoft Security Essentials, which combines both Defender malware scanning as well as active AV scanning.

Microsoft Security Essentials - Free Antivirus for Windows

Note: Upon installation of MSE, Defender will be automatically disabled. This is a normal function.

I also have Avira .... go into services by typing services.msc click on the 'gear icon' > scroll down to Windows Defender > right click on the line > choose properties, then click on the button "startup type" and disable. Stop the service.

You don't need WD if you have the paid version of Avira.