Help Me to Understand...

I'm pretty confused...I'll be straight out with it. I was seeing this girl...this amazing, beautiful, sweet, intelligent girl. Things were going great, so I thought. Then it all seemed to change up, like the flip of a switch. Suddenly, she decides that "she's not in the right mind frame for a relationship." This comes out after telling me how she told all her friends about me, told her Mom about me, sent me emails about how wonderful being around me seems...The reason I'm confused is because I shared all the same sentiments and actions. Being told one thing, being drawn in closer and closer, then being shut out for no apparent reason. Now I'm not the kind of guy who hates on women. In fact, I love you ladies, for real! The fairer sex, for sure. But come on, help me out here a bit. I'm not the sharpest knife in the drawer, but sometimes I really don't understand why or what you're thinking, and sometimes I don't think I ever will. So what's the deal ladies?