I want to leave work early today.
Did something come up? Why do you want to leave work today?
I was aggravated after a meeting I had and I need to get out of here.
I can understand your frustration after a meeting at work. If you're aggravated, it's best to leave work and regain your composure but you should make sure to talk to someone and not leave without an excuse.
I just want to leave.
I'm sure that it feels easier to leave without speaking to anyone but this could cause you to gain a bad reputation at work. If you truly are having mental anguish your boss will understand and may even help you feel better, but simply leaving will only cause you more trouble.