Why does my baby form new habits and let others go by the wayside?

Brain development: Habits are formed as the brain uses repetitive behaviors to soothe or cope with the environment. As your child gains the ability to interact with their world, and gains the ability to adapt to new stimuli, certain habits will be reinforced, and others will not be necessary. As the brain matures, it will have different needs, and will have different habits reinforced, and others not.

Trial and error: A baby will go through many behaviors as they develop. Those that are reinforced by parents are likely to remain, while those that are not reinforced will fall out. Reinforcement is through praise, involvement with the child and facial expression.