Let me quote a very famous person who grew up in poverty, Charles Dickens:
Annual income twenty pounds, annual expenditure nineteen six, result happiness. Annual income twenty pounds, annual expenditure twenty pound ought and six, result misery.
One secret to being happy with a $30k or $40k salary is to maintain a healthy perspective on your social class: If you know that a lot of people struggle to live on $18k, your $30k salary will fell fantastically great. Whereas if everyone you meet has a $150k salary and a trust fund, you'd better have the same or you will feel poor. Happiness is relative. That's the entire point of this article.
It's not surprising to find that many of the happiest grad students are people for whom a grad student salary seems quite adequate, even generous.
(Indeed, I don't believe in a general theory of human motivation. Keeping your lifestyle from creeping up to a range in which you've got to rake in a large salary to feel adequate is merely one approach to keeping yourself happy. And it surely doesn't work for everyone.)