It's just the manga way to draw him. It may be because he's older, or it shows his eyebrows are thicker than normal. I used to wonder the same thing.
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Im guessing they liked brocks eyes seems to be always closed (thats how he's drawn)
Brock squints his eyes because he and his entire family (excluding his mother) have poor eyesight. That explains why Pike Queen Lucy had a crush on him.
Brock's ethnic background is a subject of debate among Pokemon fans. He has brown hair, thinly drawn eyes (which are black when visible), and much more tan than other characters. He has eight younger siblings. They all have the same eyes, but are a somewhat lighter skin tone, suggesting Brock is just well tanned. The most common opinions suggest that he is Asian, Hispanic, or Hawaiian. The Pokemon publishers have never given an official answer to the question of his race.
No the anime just shows his eyes squinting he is clearly not blind due to the fact he thinks Nurse Joy is attractive
I've always wondered why but when he hears a pretty girl (Nurse Joy for example) talking about something that he finds romantic then he just might open his eyes. I saw it happen once and they actually look quite creepy...