AN:Written for the great maze challenge - Charlie has decided to [chill] in his new [den] for awhile, so you're on your own. You head right and see something strange up ahead. Is that a doe or a stag? Or maybe it's a patronus? That may explain why you can see through it.
It was the fates way of torturing him, he was sure of it, why else would his patronus take that form. He had seen Lily cast her patronus only once and had been impressed by the doe that had shot from her wand to join a fight against the dementors. It wasn't until the next time he cast a patronus that it happened, his patronus changed, becoming an exact replica of Lily's doe. It shouldn't affect him so much, but it did, he hadn't seen a doe since before Lily's death and his patronus was taunting him.
Lily was dead, her doe was gone, instead he had one that seem to mock him. All the happy memories he had were of Lily, although quite a few of his unhappy ones could be attributed to her as well. He supposed that was why the doe had become his patronus form, the patronus was cast using his memories of Lily, it was obvious it had taken a form associated with her as well.
He could change it, there were ways of altering a patronus, but the emotional ways were unpalatable to him. His only other option would be a dark ritual that could alter the form to whatever he wanted it to be. In the end he couldn't do it, while it was as if he was being mocked by the doe, he couldn't be rid of it. Removing the doe as his patronus would be like losing Lily all over again.
He didn't have to cast the patronus charm too often so he wasn't faced with the doe regularly, something that he was glad of. Each time he cast the charm summoning the doe became easier, seeing it was no longer such a heartache, and he could look at the doe and think of happier times.