tldr; I get a lot of satisfaction/happiness[0] from being taught by good teachers and talking to people doing research in CS and that's why I got back to academia. Not sure what will happen after this.
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I first entered industry as a software engineer after my Master's from a good university in the US. I felt the job to be very structured. By `structured` I mean - goals were well defined; best practices of SE were applied; at times there was confusion of what exactly the requirement meant but we all had it sorted since meetings with managers and product managers were regular. Research there meant to dive into existing code and/or ask experienced developers around. After having spent ~10 months, I decided to apply for a PhD program. The reason I gave to a lot of my friends at the time of leaving industry was, `I didn't have enough meat to chew`. IMO the company's structure hampered my ability to learn deeper concepts of Math-CS, and made me lethargic (probably due to the compensation). Of course, had I stayed there long enough, I could have probably defined the structure, but at that time I thought there were more interesting things to study.
I was fortunate to get accepted for a PhD program. Having an average Math background, I struggled in my first year studying mathematical models behind CS, for instance queuing theory. Everyone around me had a better intuition to such things and I felt really awkward. I survived the first semester, and in the second managed to handle more than a course.
I am currently in my second year. Most of my classes are theory based and not at all intuitive to me. However, I spend hours to understand the proofs. There are some amazing theoretical work which goes in developing algorithms[1] and concepts like these this keeps me going. Of the some I understand, I feel excited. Of the ones I don't, I hope to get my head around them some day. I meet people who are great researchers. Class projects are a great way to sow seeds of research ideas and challenging since I have to compete, in some cases, with a few classmates who have a lot more knowledge in the subject. With all classes till now, I am able to do mediocre research which aligns with the research grant given to my advisor. However, in the future, I hope to do much concrete research. My advisor knows that my current focus is on courses and he's fine with me investing more time in them as opposed to research.
Till now, all this has been challenging, and exciting. I don't know whether I'll end up in the steady state[2] of research as OP pointed out or may be I am already there :)
[0] https://en.wikipedia.org/wiki/Philomathean_Society
[1] http://infolab.stanford.edu/~ullman/mmds.html
[2] https://en.wikipedia.org/wiki/Steady_state