How do body trends start?

Like, why did so many people want a thigh gap? Why was skinny the new thing? Why is "thick" now the new thing? What's so good about trying to mould into what everyone else likes? Why not just try to be healthy? I'm not trying to say that these things aren't capable of being healthy, but people who weren't meant to have these certain types of bodies are trying to change their natural healthy bodies into something else completely. And for what? To look like every other thick, skinny, or thigh gap woman? Do whatever you want with your body, go dye yourself purple if you want, I don't care in the slightest, but what goes through peoples' mind when they try to go from an endomorph at 5'8 to bones skinny at that same height? The want to fit in? Because they want to be happy with their body? If so, why is being like everyone else so appealing to them?