Really similar to what I was thinking, you just put it better than i did :D I'd like to add to this the fact that Bungie have a top expert on human addiction working with them, so you know there's some form of system in place with the 'RNG' I think maybe it IS RNG up to a point, but when the game has given you all but asy 5 items, you'll struggle to get those 5. Then once you've got 2 or 3 of your coveted 5, the game still has 1 or 2 items you're yet to see. That's when it stops and your chances of those last golden items are reduces massivley. If the game gives you everything, why would you keep grinding? We keep going back for that last piece of the collection but we won't get it easily and they do that on purpose because we just can't help it.