It's considered controversial to be below a size 10 for women these days. And if you're a size 0 or heaven forbid a 00, you're "anorexic." And the women around you will make disparaging comments about your weight.
It's funny because men seem to like it very much. But many women don't. If my mother was alive, even she would shame me for being too "skinny." My weight was always a big problem for her whether I'd gained a bit during pregnancy or being naturally very thin as a young woman. In my early 20's I'd ask her how I looked in certain outfits and she'd say, "Skinny as hell. You need to gain weight."
I'd always found that I was at my ideal size when she'd make those remarks. That's when I always felt my most graceful, agile and comfortable in my clothes and I liked my reflection staring back at me in the mirror.
My personal test to know I'm at my ideal weight is when my tightest jeans feel like pajamas and I can sleep in them like I would a loose nightgown with no pinching or bunching.