This is a new perspective to observe my students' language learning as investment.

When they invest in language learning they construct their language system with the interaction with other people or the surroundings. 

The example "a good teacher" reminds me of one of my colleague. Different students have different standards about what a good teacher is. And different teachers have different understanding about teaching. My colleague doesn't think that she is that kind of teacher who is very active or who can have lots of interactions with her student in class. She is greatly influenced by her teachers in the university whose classes were teacher-centered and students only listened to their teachers. One day, she called me to tell me that she has to quit the English Public Speaking course because this class needs interaction and class activities. My colleague's story reminds me of "investment" of a teacher in her teaching. Some teachers are willing to invest in their teaching thus they may make some changes but some teachers are not. When the changes take place, teachers' understanding of their identity is also changing.