There is a lot of evidence to suggest that we learn to speak through immersion in a particular language.
When we are babies, our first words are most likely to be those that are often used by others in our household or those most suggested to us by those around us.
This also explains why we naturally learn to speak using the same dialect or accent used by those around us.
Although we can learn to read by memorizing and applying the alphabet and improve our vocabularies by memorizing word definitions, it seems as though actual language is learned from our surroundings.
The immersion method is also widely thought to be the best way to learn a foreign language, as well, rather than memorizing the language from a textbook.