Who wants to read what's in Celestia's private mail? Spike has a crush on a certain unicorn. This little dragon has grown up under constant literary bombardment, so it's natural for him to write out just why he cares about her. As he grows a little bit older and wiser from his contemplation, he sends a letter to Princess Celestia about his feelings and questions his own bloodline.
=#= Wrote this as an exercise in Spike's perspective on his life in Ponyville. For one thing, how he would have rationalized acting the way he did on the night of the Grand Galloping Galla. For another, how he would have responded psychologically to Rarity saving him when he was on the rampage. Overall though, this was inspired by how a crush matures or declines when one grows up. Good things do not always wither and die, sometimes they have strong roots.
Now with a follow-up story, A Kiss and A Duty.