Cough Actually, that is one of the headcanons out there: That Spike’s literally part pony.  
It’s possible dragons, being such inherently magical creatures, essentially ‘finish’ their reproduction by infusing the egg with magic (most likely the female goes second, so she can control when the eggs hatch and so no magic’s lost to eggs she can’t find a father for)  
It’d be one of the simpler explanations for why the egg did not hatch on its own, but it did hatch, into a healthy dragon, when exposed to a lot of magic; That’s exactly how dragon-eggs are supposed to work, except with dragons rather than ponies.
 
That’d mean Twilight would be either the mother or father of spike, biologically speaking, and would explain a lot of weird stuff about Spike.

 
I’m sure you are, Rarity. I’m sure you are.

Fluttertroll!!! lol

Rarity, you just game the fans more ammo to use against you.

@Background Pony  
Tell that to Bill Clinton. He’ll agree with you!

Rarity, ever head that the mouth is the source of all trouble?