The Fifth Doctor, above all others, believed the universe was a grand adventure, to be explored with arms wide open and a brave heart. As such, he didn't have any master plan to his adventures. He just wanted to go off and see places, learn things, explore... Many Doctors after him always seemed to have some ulterior motive or "end-game" that their adventures would ultimately encompass. The revived series especially plays on season long story arcs. While most stories in a season tend to appear 'standalone', they all play into one overwhelming arc that is typically resolved with the season finale (though in Moffat's case, that's not always true). The Fifth Doctor was the opposite of this. Each set of series was a standalone adventure and they very rarely tied in with future or past episodes. This isn't to say that the story was a jumbled mess but each series was its own season in a way, with a new adventure in store each time the Doctor opened the TARDIS doors. He was just as excited as the viewers about where he'd end up next.