I think the idea is that when pressed to come up with what would most impress Rose (a human from Earth) for her very first trip in the Tardis, the Doctor figured that this was the top of the list - something almost unimaginable to think of anyone ever seeing. He's definitely a "just go with it and have some fun" type so once the Steward started directing them to the greeting area/time, it was just like a "Why not?" and then on till "Wait, something's not right here! Let's figure out what's going on." You'll notice that tends to be a common pattern for the show. (Also, potential spoiler - sometimes the Tardis gets a bit wonky and they don't always end up where they might expect but instead where there might be a situation needing help.)