Talking

Is anyone else's toddler vocal but only whenever they want to be? My toddler seems to form three word sentences but some days she would rather just babble on and I don't know if it's because she has a little baby sister or if she just is very shy? I don't know if we should be worried about her speech but she knows a lot of words and understands a lot. She puts on her own shoes, dresses herself, etc. but only wants to talk whenever she wants to.