"Does your voice really need to be THAT high when you're excited?"
Have you ever been around a group of girls that just received good news and suddenly you hear screaming at a higher octave? Or been around a girl when she sees a cute baby and hear her speak really high? Or walk by a dog? Pretty much any time a woman gets excited, her voice will change. However, it is important to note that this varies between cultures. Some women only use their "chest" voice, which is considered more serious.
There are a variety of reasons that women's voices go up when they are excited, but the explanation is likely having seen others do the same, as well as being so happy that they can temporarily rid themselves of the necessity to speak in a serious tone. After all, who doesn't forget about their inhibitions when they are playing with a cute dog, or making silly faces at a baby?