The article seems to be omitting an important piece of data: Are people who make more money less happy/equally happy/more happy than people who make less? It implies by its framing that they're less happy, but that's not actually supported by any actual data in the piece (though to be fair, on cursory googling I wasn't able to find any evidence one way or another).
Now you make $(X + Y). Now some of those things become possible that you didn't dare let yourself dream about before. The problem is, you gained $Y in income, but the amount of things that became possible is 10 * $Y. You could do any one of the things that became possible, but you can't do all of them. So now they're realistic dreams, but you still can't do (most of) them.
For many people, this makes them less happy then they were, because the gap between dreams and reality got wider, even though they actually can do at least one thing more than they could before.