Dear Tanty,

Now, you’re not here anymore. I would like to thank you for being there for me for the ups and downs of my life.
It’s time for us to be apart. You have to move to Surabaya. I just want to say that
you are my best friend. It is difficult for me to say goodbye. I don’t want you to leave me but I know that moving to Surabaya is very important for your future.

Veny,

Why did the writer say “thank you” to her best friend via letter?