How do young single people become overweight?

Serious question I’m curious. I’m naturally thin and am tall and for most of my adult life ive lived an active lifestyle. This past month and a half or so I had a health issue and have been nowhere near my normal activity level and my diet has probably been a little worse than usual. Just noticed today that when I pinch my stomach there’s more to grab onto than I’m used to and I feel an immense shame and will be doubling my efforts to get back into the shape I was in two months back. I get it’s a gradual process for most people and there’s often associated issues but I really don’t understand how someone can go from normal to fat when you’re young and have no kids etc. do you just push past any feelings of shame or disgust? Do those feelings not exist to begin with? I got some body image issues for sure so maybe I’m the crazy one here. Not trying to hate I just wanna know how others have felt when putting on weight