to learn 2nd language-

What is the most important factor that affects proficiency in learning a second language?

A passion or purpose to be a proficient speaker.
IT's like a motivation.

I think there's nothing important like that. What if i don't have any purpose or passion? it will be easy to lose interest.

This motivation makes learner study constantly, doing whatever they want to improve their proficiency. It can be watching movie, listning movie, chatting with friends,, whatever..