Language is social and interactive when real needs are being met, when necessary messages are getting across, when actual communication is taking place. Adults try to learn language isolated from people, or they are often placed in contrived rather than real communication settings. Little children never learn words or sentences apart from people, or apart from what's going one. It is always in context and always real. It also turns out that way for them in a new language.
Children are completely open.
The world they know is basically friendly. Their guard is not up. They don't have to defend themselves against it. This makes them completely open to language input. Language must get in before it can come out.