In contrast to the way a child learns, an adult has had years to master at least one language. An adult can weigh and compare new concepts to learned knowledge. As adults, most of us do not require someone else to motivate us into learning something. We are aware of our interests.
Grammar is necessary for learning the mechanics of a language, but it is something that can be overdone in some language courses. Adult students don’t need to spend too much time in grammar when learning a new language, unless it is for the purpose of mastering the linguistics of a foreign language. In English, for example, the subject in a sentence is determined by the order in which words are dropped into a sentence. In Japanese, morphology determines the subject sentence structures. Once the ability to see patterns in a languages is acquired, the adult student does not require endless hours of grammatical studies.