Why are the teacher angry when the student can't do something in thelesson?

This is a question that always make me want to find out the answer and this question make me want to flashback to my experiment when I taught my Students in vocational school.

After passed S1, I taught a students in vocational school. In the process of teaching, I gave all thing, all my knowledge that I can give to my students. I tried hard to make them understand english lesson. Sometimes I used game, I also asked them to have a role play, or group work. And sometimes I also taught them by using metode ceramah or teacher centers to deliver the material.
When the process of the taching, I ever wanted to upset to my student because the student hasn't understand the material that i delivered.
But I didn't said to the all students that I was upset at that time.
Because I looked at my self , maybe there was something wrong with my teaching strategy or something else.
And I was ever sad when i saw the students result of examination have bad score after count the average of all students.



I can control myself at the time, because i want to teach them rather than to scold them. I just want them to understand the lesson.



Now, i'm being the student again. And i feel when the teacher give a feedback such us angry but not angry i think ,it is maybe more emphatic.
It is not comfortable.
Then, i reflect myself, maybe i ever conscious like that to my student. Unconscious angry or more emphatic, so i feel also what they have ever feel