I'm probably on IP because of depression, at least indirectly. I took a job that ended up being really stressful (the boss was a J-E-R-K and also a bad manager), and I had to resign it because it was so awful. I ate poorly and drank a lot while I was working at that job, because eating take-out and junk was my default behavior when I was too busy and too stressed to have time for self-care (which cooking healthy foods is totally part of, right?). And the culture at that place was kinda messed up; heavy drinking was part of it, probably because that manager stressed everyone out.
And then I beat myself up for making the mistake of taking that job, after I quit. I got super depressed. (While my family has tendencies toward real depression, I think this was little-d depression, the kind that is short-term. I don't think it was real depression.)
And you know how sometimes those depression checklists list "sudden weight loss" as a sign of depression? Yeah, not for me. I gain weight. Rapidly. Because I self-soothe with foods I find comforting.
Anyway, yeah, 20+ pounds later, I came out of the worst of the depression fog and didn't recognize myself. And I started IP (for a second time, but I did the first wrong in several important ways; don't let my reboot be a discouragement, please!). I'm well on my way to working through the depression weight, then will come the hellboss weight, then will come the rest, which I gained more slowly through somewhat less dramatic bad habits (the last place I lived had some great restaurants

); I expect that will take a while, but it's OK. I can do it!
Anxiety and maybe a mild case of obsessiveness is why I'm doing IP instead of just calorie counting. I'm scientifically minded; the math of calorie counting never bothered me. But I would get really anxious about whether I was measuring right, "how many servings does this recipe make?", and the only way I could calorie count was to eat only processed and chain restaurant food. Even fruits and vegetables stressed me out. "Is this a medium banana or a large one? I don't know!" ... I know that sounds really stupid, but it seriously affected my mental health.
IP is easier on me, because 1) I know if I get really hungry I'm allowed to have another packet, so I always have insurance. 2) I know they build in a buffer for carbs, so if I accidentally get too many or too few veggies in a day, it probably won't hurt me in the long run. I can trust them to have been obsessive for me.
(Sorry, I didn't set out to write a novel, but there you go.)