When I entered grad school, I noticed that the older grad students just seemed... oddly bitter? The 6th years were living in a different world from us 1st years. Some of them would joke about it, some would deflect when asked, and some students we only heard about. I always wondered what made them so. Does their love for science slowly leave as they pass their time here? Will I become that way?? How do I avoid that?
Rest assured, most of us still really love science. We're fascinated by what we study. It's why we came here and we stayed on so many years. For me, if anything, I have become more and more fascinated by how animals coordinate their limbs. I love thinking about it and talking about it. I do still feel lucky to get the chance to work in neuroscience full time.
So let's start with an obvious one: we're not paid well. A decent STEM PhD student stipend in the US is about $40k. Note that many schools pay below this. In a city, about 2/3 of that goes to rent and food and other living expenses. Many students have college loans they are paying off with the remaining 1/3. You can save some, but in the grand scheme of things it's really not much money, especially in an area where a "cheap" condo goes for $1M. By the time you're in your 6th year, many of your college friends who did not do a PhD have mostly settled to nicer jobs and are living a proper life with their salary.