The point being, that if you can't show that you're a genuine visitor, they'll think you're trying to overstay--it's not hard to 'miss' your return flight home. We know you're being honest, but the IO really doesn't know any of that; they ultimately have to use their judgement when you present yourself at the border. They've heard the 'I'm just visiting' thing before, and it makes it difficult sometimes for genuine visitors to get in because people have abused the system before. Sometimes without some idea of what you're going to do and see while you're there, they'll just assume you never had an intention to visit--that you'll overstay, especially when you're young and don't have loads of obligations back home. Like Vicky said, sometimes they can get suspicious when you say you're just going to stay with a friend and bum around and look at stuff when you're asking to stay for such a long period of time--many visitors only come for a couple week, see some sights, and then bugger off home.
You may want to sort out some sort of itinerary to show you have solid plans of sightseeing and doing tourist type of things, and show that it will be for however long you're planning on staying for.