One possible version:
Dear Tom,
I’m so glad to receive your letter!
As you know, now I am faced with a choice between arts and science. It was not an easy choice for me to make. After careful consideration, though ,I decided on science as my main subject of study. The reasons are as follows:
First of all, science graduates are in greater demand than arts ones in China. Then, the major in science can help me achieve my childhood dream of becoming a scientist. Besides, my parents wish me to study science at college because they believe I will have a brighter future as a science student. Last but not least, my physics teacher is my role model, whose influence on me had a lot to do with my final decision.
It is for these reasons that I finally made my choice.
Yours,
Li Hua