I don't share that experience on Android. If you exit the Appviewer, and later open it through the list of open applications (it is the button with a square symbol on my Android), you don't need to login.
If you setup your phone to aggressively kill apps in the background, or open it again via the Appviewer App in your launcher/screen, you are requested to login again. I prevented myself from doing that by removing the Appviewer App from my start screens; so the easiest way is to open it via the mail/text link or the running app overview.
In the case your phone is running low on resources while running your other apps, it might kill Appviewer to free up some memory. However is has been a while that I have seen that happen on my few year old Nexus 5. That seems to have enough memory to never kill apps in the background with my usage pattern.
Do you still see the same if you open a new app and return to Appviewer via the running apps/multitask/square symbol button app overview?