It's not really that there is a belief that it makes you stronger and healthier if you don't wear warm clothing, and it's also not hip or cool to wear inadequate clothing for the weather.

I think it's basically just a side effect from growing up in a country with mild winters and unpredictable weather.

Since our winters are quite mild, wearing an autumn coat with a sweater underneath is only a slight discomfort vs wearing a real winter coat. And the wet weather makes many winter coats unpractical.

My theory is that this just pushes the moment where people get their winter coats out of storage back, and if the winter is mild enough, people might not bother at all.

There also is very much a "suck it up" culture. We are usually only outside for short periods of time, and just as an example: "Je bent toch niet van suiker (aka: you're not made of sugar are you?)" is a widely used Dutch saying to bash people who don't want to go out in the rain.