I am about to cry. What if it was Spike’s mother who wish to have a child and when she travel to do her mission, Sludge abonded their egg, because he didn’t want to be dad. What if Spike’s mother found out what happened? WHat if the story Sludge told was based on what really happened and later he lied to make it more dramatic. And lied in the end again, because he still didn’t want to be dad. Poor Spike. Why is his dad so mean?

The fact that Spike’s father Sludge is so mean is really sad

What about Spike’s mother? Where is she. Is she dead?

@Ryodraco  
He abandoned Spike as an egg.  
He learned Spike’s fame and by somehow connecting the dots, he found out Spike is his son.  
He reach Ponyville to use that connection to get some riches, but crashed right next to his son.  
He was suspicious of ponies’ intentions at first, but then they accept him as their guest.  
He could tell them the truth, but was afraid of their anger in case they know he abandoned Spike.  
So, he didn’t tell them so they treat him kindly, and not going to tell till end of his guesthood.  
Only when they decide to sent him away he admitted that he’s Spike father, to stay there.  
He make up a story so they wouldn’t kick him out, and they let him stay there with Spike.

@orbsah  
But then why wait until after he he learned the joys of pony coddling before he revealed who he was? Clearly he doesn’t actually care about Spike’s happiness, so it looks a lot more like a idea he thought up to stay in the castle.
 
Note that my main contention is the above rather than other details, simply because I do think its possible he could have inferred Spike was his if my idea is right that Spike is pretty much the only dragon his age right now (dragons are not common, and every other dragon we’ve seen has been older than Spike).

@Ryodraco  
Holes in his story could be explained away if he abandoned Spike as an egg.

Nah, there are too many holes in his story. Heck, aside from the ones the Mane Six and Spike pointed out (before proceeding to believe him because if they didn’t we wouldn’t have an episode), we outright have him make sure he comes up with an explanation that involves a location the ponies have never been to or heard of and thus can’t try to confirm.

He does look a LOT like him.
 
Except Spike is much cuter, of course.

@punzil504
 
Thought the same thing

I feel more sorry for Smolder…I mean, anyone here been in that one situation as a kid, where you at a friends house and their parents are yelling?

@Lynx318  
i think there’s a fanfic of that concept

Like anypone would approach him to take a DNA blood sample for paternity test.

probably

That would be interesting