Different jobs have different features. When high school students choose their majors, only a few of them have a clean plan for their future career while others only choose their majors according to their hobbies or interests. When all of us enter the job market after graduating from university, we find that the decisions we made before are very different from what we have thought (I adjusted your sentence with the same pronoun: “us”, so it remains coherent, but be careful because in your original sentence it was difficult to understand who was “they” and who was “us”). For example, a student may think that if they learn medicine for over five or eight years, it's boring, but when they find that doctors can easily get promoted as they get old and have more experience, they regret having such hard feelings toward their decisions. As for people who want to be teachers, they find that they careers are very different from doctors. The knowledge they have gained from university can't help them becoming competent at their job. They need to empty their mind and learn new things all the time as experience don't work for them. Only new knowledge is necessary, even when they get old, they still need to learn new things and only if they do that they will become good teachers.

Different jobs have different features. When high school students choose their majors, only a few of them have a clear plan for their future career. The others only choose their majors depending on their hobbies or interests. When all of us enter soceity after graduating from universities, we find that the decisions we made before are very different from what we had thought.For example, someone thinks that if they study medicine for over five or eight years, it's boring,but when they find that doctors can easily get promoted as they get older and have more experience in their job, they feel regret about their decisions they have made. As for the people who want to be teachers, they find that their careers are absolutely different from doctors. The knowledge they have gained from universties can't help them to be competent in their jobs. They need to empty their mind and learn new things all the time. Experience doesn't work for them. Only new knowledge is necessary, even when they get old, they still need to learn new things. Only doing that can help them to be good teachers.