Having a teen niece with a baby and having worked with teen mothers, I can tell you that the novelty of a baby wears off. The pregnancy attention is great and the attention a new young mom gets when baby is born is great. Then, slowly, the friends stop coming around, start making plans that don't include you, and you're kind of stuck alone.
So, my guess was that having Caylee was fun at first because it was like having a living baby doll. Then, when Caylee started taking away from Casey's life, when Casey saw she was missing "La Bella Vida", when Casey's friends lived their lives in fun ways without her, she wanted that and knew she couldn't do it with a baby. Resentment towards Caylee started to grow. She could put the false face on for her parents but the resentment was probably building. She knew she could have "La Bella Vida" sans Caylee and decided to do what it took to get to that step.
Of course, giving up Caylee for adoption wasn't going to work because Cindy and George wouldn't allow it. And she wasn't going to let her parents control her kid ... so she did what it took to be free from everyone.
At least that's how I see it ...