You’re pretty excited when you graduate from pre-school, even if you don’t really know why. You get excited for graduating elementary school because you think middle school will be exciting as heck, but then it’s not - your excitement increases because you want to get the heck outta there. You yearn for college and for partying and relaxation, which you falsely think will continue after graduation.
That’s when reality sets in, you have grad work to do, and you realize that you’re sinking into debt. Now, the slope after grad school honestly just depends on your major in college/career. The one in the graph applies to most people working in Humanities-related careers (unable to pay off their debt. A slightly less dramatic negative slope would apply to people able to pay off their debt (upper-middle class citizens). A negative slope for a short period of time, followed by a dramatic upward slope would probably apply to business tycoons/inheritors who are initially disappointed by having to work but then realize that they enjoy bathing in money.”