As someone allergic to grass, I'm in the same boat. Would love an explanation as to why "touch grass" has positive meaning.

When you tell somebody to touch grass, you're basically telling them that they're becoming overly swept up in something on their screen to the detriment of their mental health and social wellbeing. It might as well be "go call your dad", because the gist is just for you to remove yourself from a situation that's clearly causing you to disconnect from reality, either to your detriment or the annoyance of people around you.

Like that's the baseline that's relevant whether you're saying it to some kid screaming in a video game lobby or some middle aged dude who's going off his rocker in the facebook comment section of his local newspaper. 'You're embarrassing yourself and you need to get some perspective, because the things you're saying don't make sense, and you'll see that if you walk away from the machine for a bit.'

It means leave your gloomy room where the only light soures are screens and your only (social) activity is on the internet.

The person saying it is discrediting what you're talking about as non important.

What they really mean is:

"your problem is not important to me"

Because they dont understand your problem, different people have different problems, the people saying it usually have a very narrow/priviledged life.

Pretty sure it just means "touching grass", as in "step away from the computer, walk outside, put your hand on a patch of grass". Previous generations might say "stop and smell the roses", although that idiom is generally less literal.