Dissatisfaction: it's what's for dinner. That is, if your automatic kitchen is doing the cooking and you yourself have nothing to do. In "The Veldt," thanks to technology, people are unhappy. Lydia Hadley doesn't work and feels useless; George is so unsatisfied that he's smoking and drinking too much; and the kids are so dissatisfied with their parents that they've found another parental figure. (Spoiler: it's the nursery.) We could say that, in "The Veldt," dissatisfaction comes from the effect of technology on the family. But this story isn't just about some made-up technology; it's very much about the 1950s, when people came home from World War II to discover they had money and lots of stuff, but weren't always happy.
Bradbury's characters feel dissatisfaction rather than something more extreme (hate, depression, etc.) so that more readers can relate to the emotions.
Bradbury presents dissatisfaction as something that can and should be avoided. We can all be happy, if only we know what we really need.