Look, I'm not saying I hate air, but it does present certain problems.
The advantages are obvious. The ability to fly kites, appreciate clouds, watch pretty sunsets, and oh yeah, breathe, all have their roots in the existence of this ocean of air over our heads.
But for astronomers, it's something of a problem. One of the biggest issues we have with air is that it moves. Little packets of air kilometers above the ground swish to and fro, and that distorts our view of the heavens. These little packets act like lenses, bending the light from stars and other astronomical objects as it passes through. This causes the image to jump around, blurring out details in the telescope (and causing stars to twinkle). For historical reasons (and sometimes, I swear just to confuse people) astronomers call this condition seeing, and when you say "the seeing is bad" you mean that things are so bubbly up there that observations are compromised.