Read the following letters and answer the questions.

5 October 2017

Dear Melodi,

As you already know, I have recently finished my postgraduate degree and will be leaving for Denpasar at the end of this week. Please consider this to be my sincere goodbye to you.

    I want you to know that I am leaving here with truly mixed feelings; happy about finishing my education, but sad to be leaving the school where I have had so many unforgettable moments with you. The last two years as an English teacher at our great school was the best period of my career so far. I learned a great deal and worked with you, who, I am sure, I will remain a good friend for a long time. I can only hope that our friendship will never end.

    Thank you so much for making my time at the school a truly enjoyable one. Let's keep in touch. Speak to me before I leave on Friday and I will be happy to give you my new phone number and e-mail address.

    My very best wishes for the future go out to you.

Yours sincerely,

Dona

What made Dona write the letter?