Well, I probably never have gotten as extreme as your girlfriend- I'm an INFP sp/sx, after all. But it felt extreme to me. And I also have bipolar disorder so that muddies the waters a bit. I'm married to an ESFP 7w6, probably so/sx, so he kind of loves it when I'm like this.
Why do I get like this? Shrug. I guess I have strong opinions, and it's kind of like flirting. I'd see it more like- 7: playful and 8: assertive and straight forward. I don't draw people into my world so I can crush them like idiots. I don't know your girlfriend so I can't speak for her, but I wonder how much of this is your perspective and how much of it is actually how she is/was. I relate more to what you say about your ESFP 783 friend, and I'm no ESFP 783.