I am doing research about English language learners. I want to know why learners want to improve their English, if they already speak English proficiently.

Maybe they want to be 'better' not 'proficient'. by the way I don't think there be a clear definition about 'proficient '

I'm sorry. Proficient is not a good word in this case. Do you want to be able to sound more like a native speaker. Would you like to be able to use English idioms or phrases, such as " up the creek with out a paddle". Would you like to learn to be able to speak more fluently.