Why do girls do that? ?

So my friend ( who is NOT going to be named) is always making b*tchy comments on basically everything. I"m a REALLY good baker and I made this mud cake that was yum and super moist. Guess what she says? " hmm, you"re pushing it, it"s a little dry". I got 100% on my science exam (woo!) and she says that it was "just a lucky time" which is not true since 96% was the average of all my exam marks. Why is she doing this? It"s so so so irritating, I swear I could just slap her stupid face face silly.