The explanation is coming up next chapter. I've embedded hints of it a few times, but it doesn't seem as though a lot of people have picked up on the "why" as opposed to the "who."

I was kind of a loss as to what to do with this chapter, for a while. I knew the search had to take the whole chapter, but I didn't know how to stretch it out that long. Using to demonstrate their reliance on each other turned out to be a good choice.

Draco probably wouldn't have survived a few months ago, before he got to know Harry. Now? It'll still be a struggle.

Next chapter will probably be up day after tomorrow.