How do we learn new languages? To answer that question, maybe we should ask why we developed languages.
One theory is that language was first a tool for cooperation. Babies, while growing, learn very quickly by copying and imitating parents, picking up many social cues, including body language. They also learn shared social conventions and beliefs of the culture. Language development is part of fitting in–meaning, as an aside, that language does change a learner’s view of the world.
As babies learn, their minds begin to recognize new patterns. Learning anything requires the mind to change and adapt to new information, by storing, processing, and recalling knowledge, and patterns are essential for organizing information. Grammar is learned through a process of trial and error, with children applying patterns they know to new situations and refining those patterns as they learn.
For second language learners, the same process is followed, but at a slower pace. Previous patterns from first languages literally get in the way of learning new patterns, which are often incorrect at first and must further be refined. As a side note, this also means that error correction, especially through appropriate and correct example language, is also an important part of learning languages.
Why, then, do new learners make mistakes? Unrefined patterns lead is to produce incorrect sentences, and only through learning more can these patterns become similar to those in native cultures. This means that people will always make mistakes; essentially, people learn best by making mistakes.