Outlook has some built in systems to help itself run more smoothly, including the ability to enable or disable Add-Ins based on the amount of time they take to load.
If applications exceed this expected load time, they will become disabled on Outlook Startup.
Even after enabling the Add-In, this process will run again the next time the program is started which may result in needing to enable each time that Outlook is run.
Each time Outlook is opened, the ViewedIt Add-In is not there to use. Even if you re-enable it, the next time you close and open Outlook, the Add-In is not there.
The user does not have permission to enable Add-In permanently or Outlook has deemed that the Add-In is loading too slowly and has disabled it.
If insufficient privileges:
If Outlook is disabling automatically: