Explanation!

So the letter dan writes is

For y/n. Remember when y/n told him to write a journal and name it after someone he loves? Yes, so Dan started journaling and he name the notebook y/n so as if he was writing letters to Y/n. And yes, Y/n dies in the airplane because a hijacker shoots him. Dan found that out from Y/n's mother and refuses to believe it was you. He waits for 5 years for you to come back but you never do so he accepts the truth because he knows if you were here, that's what you would want. So his gift for you on your 25th birthday is to move on and stop writing in that journal and go pursue his dream.

Thats it!