Happiness has a way of leveling out over time. Everyone returns to their baseline happiness, other wise known as "set point". So when you first go driving and you have the money, security and such........your happiness spikes. After sometime, people return to their baseline happiness. The theory is called hedonic adaption or hedonic treadmill.
Since majority of people like to complain, it does't take that long for people to forget where they have come from and just dwell on the negative rather then celebrate what they have accomplished. Human nature is to complain. These same people (which I include myself to a degree) could have a job sleeping on a white sand beach, at 70 degrees, cool breeze, making 200k a year and will still "whine" about their day at work.
My opinion anyway.