The most well travelled person that I have ever known visited about 70 countries. That constitutes having visited roughly 35% of the sovereign nations in the world. He was not a young man, and he had many stamps in his passport before he became an adult. It might be possible to set foot in every country in a single lifetime, but many countries are rather extensive and it would take a good block of time to get a real feel for each place. I personally wouldn't invest the time and money required to check a country off the list unless I could really say that I'd been there. I don't think that visiting an airport or a train or bus station, or simply driving straight through a country qualifies as having actually been there. It would take nearly 4 years to spend a week in each country in the world. And of course, some are not easy to visit. Flights, train and bus tickets, rental cars, accommodation, food, visa fees, would all add up. It would be an enormous undertaking and incredibly pricey.