The ground might be too rocky or hummocky or densely vegetated. Once you've spotted a possible home for the night, lay out your groundcloth and lie down to check out the slope and whether there are big protruding rocks that will poke you all night long. Look for overhead dangers. Choose sites that will drain well, even in a downpour. In dry country, avoid flash-flood zones, like the sandy creekbed of a canyon.
Mosquitoes are worst on a warm, humid night, especially if there is no breeze.
On very windy nights, you'll want your wind at the back of your tent. If possible, hide in a clump of bushes, behind a rock redoubt. When storms threaten, give up comfort for protection choose a protected spot over a flat one. A calm evening can become a windy night. Set up your tent right the first time, with firmly planted stakes and taut guylines.