The Google Daydream may not be available on the Samsung Galaxy S8 and S8 Plus because the devices exceed the requirement of Google's VR.
When installing the Google Daydream on the Galaxy S8 and S8 Plus, an error message will appear saying "compatible version is missing" and will not be able to download Daydream. According to Google and Samsung, the Google Daydream will not be available to Galaxy S8.
The Samsung Galaxy S8 is the most capable mobile VR system since it comes preloaded with a trigger to install the biggest mobile VR platform - the Oculus-powered Gear VR. However, Daydream offers more access to Google Play which includes YouTube application.
The other thing that stops the installation is the single line in one of the system permissions files. Accessing this system permission file requires root access which is not available for Galaxy S8 as of the moment. Google only support this line of code to devices that meet the Daydream specs. It also involves Samsung and Google distribution agreement, so it will not conflict the current Oculus relationship. Eventually, the Google Daydream will be available on the Samsung Galaxy S8 and S8 Plus.
Google Daydream can also be installed manually if the phone is not approved by Google. However, the device should be operating Android 7.0 Nougat. Also, it must have root access enabled on the phone, Google's Daydream app and Google VR must be installed, and Root Explorer app from the Play Store is required.