Well, I never lived in a particularly fat-positive environment as a child, but generally people didn't treat fatness as a terminal illness around me either. Even in school, the topic rarely came up. Sometimes people would talk about how this person or that person died from "being too fat," but it was all anecdotal; no causes or effects were provided in detail, so it didn't effect me that much.
I had a little bit of nervousness about my weight gain briefly in high school, but it passed quickly. It made me very nervous when I started having blood pressure issues in my mid-to-high-30s (I thought it was my 40s at one point, but thinking back on it, I'm sure I wasn't 40 yet when this happened,) and my GP told me I'd never be able to lower it without drugs or weight loss. Well, guess what? I lowered it without drugs or significant weight loss. I started going for walks more often, and had more popcorn/seltzer and less sugary snacks, and as a consequence, temporarily lost 3 pounds, and reduced my blood pressure. I was just experimenting to see if I could find something that worked, and it did.
I think it would be going too far to say I've been fearless since then, but it taught me that I didn't have as much to fear as I'd thought I did, and I've been far more comfortable with my gains since then.
I have to say I never had goals. I just met someone who really loved chubby guys and, after initial resistance, just kinda stopped being so careful with food and HAVING to exercise 6 days a week. At first it was slightly alarming to watch my body change, but she was just so sweet and reassuring and over-the-moon about it that I soon forgot about reservations and let it happen, and had fun with it. When we went separate ways I naturally lost weight over time. I've also always been a sort of confident person.