Because people are willing to pay more. Why should I make money because my house is worth more than it was? Because people are willing to pay more. There's no more "should" required than that.
Um... if the only legal tender around was an asset with fixed supply, people holding it wouldn't be willing to pay more, (or invest it in making more stuff) that's the whole point.
But you see my point, right? I'm challenging the question of "why should someone buy that thing for more than the seller paid for it?" The answer is: because the new buyer is willing to pay that for it.
