The most common way people visit all 50 is to visit different states along their life journey. During childhood states were visited during trips to visit out-of-state family and family "road trips". During college-age years, people branch out and travel to new places for school. They meet people from different places which gives them a reason to visit new parts of the country. Others join the military and travel to bases across the country and the world. Careers take people to places they never expected to go. As adults, vacations provide a break from the normal routine. Retirement, for many, offers more freedom to travel.
Somewhere along the way we start counting how many states we have been to and then make plans to go to the rest until we eventually go to every state in the U.S.A.!