Why does alcohol, such as bourbon go "flat" when you leave it sitting in a glass?

I've noticed that if I leave my bourbon sitting in a glass for a while, it will become cloudy and will taste awful. It will not have any alcohol.

I am assuming that the alcohol evaporates, but I haven't gotten a clear answer.

Also, the particular bourbon that I bought recently went "flat" really quickly, whereas others usually take a little while. Is that a sign I am drinking lower quality alcohol or does it not really mean anything?