I grew up without much money and knew a lot of peope who didn't have any money themselves, and I can tell you that it's a huge burden. When you are constantly trying to play catch up and running out of money, things can get very hard and stressful on you.
It makes them feel more normal. If you're poor and everyone
around you is wealthy you feel really impoverished. If you're poor and
everyone around you is poor, you feel typical or average and it helps you
cope with the misfortune. It makes them feel that they are not alone in their misfortune
Impoverished people are likely unhappy that they can't afford, or at least easily afford, needed things such as food, clothing, power, water, and a home. I wouldn't say they are unhappy in general - just unhappy to be living in poverty. I don't think anyone would want to live in poverty if given a choice.
It's not a pleasant thing to have to worry about money,food and shelter on a constant basis, while I am sure some impoverished people are happy I'd say the vast majority of them are stressed and concerned on a constant basis while they try to provide for themselves and their families.
People that do not have as much money are not necessarily unhappy. They have their good days and bad days like other people but they just live differently. They live on less and expect less in return. In some ways they may be happier since they don't need the material possessions other people need to be happy.