You're only squinting because you've developed a habit with sunglasses and aren't accustomed to the light levels. As I said, creating a problem for yourself. 
Don't own a pair and I can see just fine in any kind of sun, driving included.
Northern Europeans have evolved to have poor tolerance to bright sunlight on sunny days. We live in a part of the world where it is cloudy/overcast on the vast majority of days.
Aside from this, UV exposure increases the chance of eye-related issues later in your life (whether corneal burn or sunburn to your eyelids potentially causing eyelid cancer).
I get that you can increase your tolerance to sunlight but there is no way sunglasses are unnecessary. If you walk around without shoes all your life then your feet will toughen up but you'll be more prone to foot/ankle damage eventually. Same thing.
