He didn't have adventure thrust upon him. He went and decided to do something and then did it. However badly it backfired, it would have been (more or less) compatible with the houses' character for Slytherin to be for the people who make grand plans. For good and for evil.
Though what I was pondering earlier -- that if the house represented virtues, Harry would have needed a more varied set of friends -- would be made even more true in this case. Harry, Hermione, and Ron desperately needed such a good planner, Slytherin or not, in Deathly Hallows; they spent so much of the story running around England in flight from the bad guys without actively planning how to search for the Horcruxes. Go to I Know Not Where, Bring Back I Know Not What without the magical bride who can turn into a bird and also advise you where to go to do just that.
