It's impossible to say what kind of life she would've led if she hadn't been the Chosen one. I for one don't think she was, even before, ever a shallow character. If we judge from the first season she seems a bit unsure of herself, but ultimately seeks out Willow as a friend instead of Cordelia. Of course, she does already know about Slayers then. But I think we can assume that non-Slayer-Buffy would have matured eventually and not stayed 15 forever.
I'm not really sure if she had been happier. In this life as a Slayer she got a sister that she loves. She would've been all alone as her mother died as non-Slayer-Buffy. Ultimately I also think being the Slayer was Buffys purpose in life. I like the little speech she delivers to the potentials - that most people are not even sure what their meaning of existence is and they never find out what is the sense in life. At this point I think in season 7 Buffy is proud of being the Slayer. Even though it is a burden it is also a gift. A Slayer has a purpose. Even if it is her death. She probably would've been happier, but her life would have not better, imho.
I'm not really sure if she had been happier. In this life as a Slayer she got a sister that she loves. She would've been all alone as her mother died as non-Slayer-Buffy. Ultimately I also think being the Slayer was Buffys purpose in life. I like the little speech she delivers to the potentials - that most people are not even sure what their meaning of existence is and they never find out what is the sense in life. At this point I think in season 7 Buffy is proud of being the Slayer. Even though it is a burden it is also a gift. A Slayer has a purpose. Even if it is her death. She probably would've been happier, but her life would have not better, imho.