Hello, my name is***** and I have over 20 years of experience as a veterinarian. I understand that you are concerned about the amount of sneezing that your girl has been doing the past 6 months, especially when she is excited.
A sneeze can mean that something is irritating her nose. The irritant can be dust, pollens, an irritating, strong smell, increased tearing draining into her nose, a viral, bacterial or fungal infection or a foreign body, a tooth root infection or nasal mass irritating the nasal mucosa.
If this were a one time thing then dust, pollen, a strong smell, or increased tearing would be most likely. Since this is a more chronic thing I would be more inclined to think of an infection (fungal, bacterial or viral), a foreign body (blade of grass, a grass awn), a tooth root infection, or mass in her nose.
If she has no, or a little clear, nasal discharge, is eating and drinking normally, and her nose conformation is unchanged (no swelling or bumps) then it could be related to a viral infection, but in that case it should have passed on its own. 6 months is much too long for a viral infection to linger.
The next step diagnostically would be sedation, dental radiographs to look for bad tooth roots, a nasal scope and skull radiographs to look for a foreign body or mass. It may be helpful to collect samples for bacterial and fungal cultures as well.
In the meantime it may help her to use a humidifier in the room she sleeps in most of the time, and/or taking her into the bathroom with you when you run a hot bath or shower to breathe in steam/humid air to soothe her nasal mucosa.
Some sorts of "sneezing-type" behavior is actually social behavior. Dogs will display this sort of behavior when excited, greeting a friend (human or dog) as a way of showing they are excited and want to play. With these pups the "sneezes" really only occur in a social context, and they tend to be a very "mild sneeze" rather than a full nose clearing type sneeze.
Please let me know if you have any further questions.