I'm using a Roku Express that's about a year old. Up until a month or two ago, I never had any problems but since that time, I have to reactivate/log in to Youtube every other day, sometimes daily. Before this started happening, I was logged in for months at a time without issue.
Anything I can try?
Log in to YouTube. Go to the device connections page https://myaccount.google.com/device-activity
Remove your Roku from that list. It may not be listed, or it may be listed multiple times. Make a note of that.
This is also a good opportunity to remove any other devices that you don't want to be logged in to.
On your Roku, remove the YouTube app.
Reboot the Roku.
Add the YouTube channel to the Roku.
Log in. See if the behavior continues.