Why does my coworker, who shares my office space, talk so extremely loud; all of the time?

She does not have a hearing loss. She also talks over me and others.

That is the sign of a person who wants approval, and attention. Show them a little love, and use a hand gesture the next time they get loud to "keep it down"

Are you louder? Is she merely trying to drown you out?

she might be in denial of the hearing loss or she might feel like no one is listening or shes always done it so she feel heard by others, because she feels insignificant or ignored , if you want it to stop, listen to her as attentively as you can and if you feel she is talking to loud gently remind her that you are in the same room as her and she doesn't need to talk that loud but do it politely and look like your listening

Because she has diarrhea of the mouth. She probably didn't get enough attention as a child.

She is doing this because she is insecure, and wants to make sure that everyone around her understands how "important" she is. Obviously, she is NOT, but she needs reassurance, or she wouldn't do it.

I took my son to the doctor a couple of years back because he was practicially yelling all of the time, I mean this boy was the loudest talker I'd ever heard, and I am naturally very quiet. I though he had some hearing loss or wasn't developing correctly in regards to volume and control. The doctor told me that young children as strong opinioned as Derrick somehow think that they know it all and are the smartest beings on this planet and as a result of this, they love the sound of their own voices and are just very loud people/ And this was when he was 3. He's learning volume and squleech control, now, and he really is a smart kid. Your co-worker is putting herself (in her own head) way above everyone elses mental capacity. She thinks she's brilliant, and everyone needs to hear it.
Either that, or she's constantly checking to make sure her voice still works. lol

Insecure. Wants everyone to pay attention to her. As her, why do you want everyone to pay attention to you pn the phone. We all know you came to work.

Sounds like she isn't aware of her behavior.