Bobby cries in response to the balloon because of classical conditioning.
Classical conditioning is a term coined by Ivan Pavlov in the 19th century. He experimented on dogs, trying to elicit a particular behavior from them. Each time he gave the dog food, he would ring a bell. Soon afterwards, the dog would start salivating whenever it heard the bell because it knew food was coming. Bobby is somewhat similar in that he starts crying whenever he sees a balloon because he thinks it will pop and make him scared again.