In your opinion what is the most important factor that affects proficiency in learning a second language?

In my opinion what most affects proficiency in learning a second language is the will or motivation to learn that new language. Of course there are other factors or characteristics that affect the succesful adcquisition of a second language like intelligence, age, opportunities to practice your speaking, etc...But for me it´s obvious that if you don´t have the desire to learn you probably won´t dedicate much effort or time to improve your knowledge. And that can be extrapolate to many things in life.