The best way to motivate yourself to visit all countries in the world is to make a list of them and cross them out one by one until you visit all of them. For the sake of convenience, I have made a simple app for that.
The idea of this app is not unique, but it is special in its simplicity and absence of useless functionality.
The app is not free, since it uses payable google storage for storing your uploaded photos and data. On the bright side, your saved countries are accessible throughout all platforms. In addition, you can see all other users of this app, who allowed to be visible, and see how many countries they have visited.

Enjoy Travelling.