why am I nervous of doing what's right?

I was on the bus going home today, sitting right at the back of the bus. There were two girls in school uniforms sitting somewhere infront.

A teenage boarded the bus, and stood very near one of the girls despite there were so many many empty seats available. Then, the teenage was rubbing his groin against one of the girl's shoulder. Oh man, that's disgusting. He continued even though she pushed him away a little.

So I go to him and ask him to stay away from the girl. Then, he go to another girl and do the same thing. I said: "what are you doing? go away". Then the teenage got off the bus on the next stop.

Then I realized my heart was pumping very fast. It's not the first time I stood up for others who are strangers. But why do I feel so nervous to do something right? Feel so coward,haha

But realize it's been the same for many people, that we all felt nervous to do a right thing for someone we don't know.

Even so, I hope I can overcome this kinda feeling and simply do what's right without feeling weird or nervous next time.