Probably because she's your employee, and not your pal.
The two of you are on unequal footing. You have a ton of work advantages that she does not. More $$ and more coworker respect are two of them. Your resume looks better than hers does, too.
If you snap at her for any reason, even if it is 100% not justified, she cannot respond in kind without legitimately fearing for her job.
In any event, accept that you are not going to be palsy-walsy anymore. Instead, she is your employee. This means you treat her professionally and courteously, but you don't confide your inner secrets to her, etc.
For a lot of younger/newer managers (and I don't know if you are, but your post sure makes it sound like you are), this is one of the hardest things to get used to - you aren't in on the water cooler stuff anymore.