What makes pokemon get along, or not get along, at daycare?

Question

This making me wonder, I mean, almost all pokemon I breed, never gets along or like each other, only time I remember they do get along is when I breed vulpix and eevee together. I ain't really wanting to make it more effective to get eggs, or ect, just curiosity on how it works. How do they get along, or not get alont? Is it the kind of pokemon they are, the types, or does it has something to do with the nature?

Answer 1

Breeding rates

Same species, different ID numbers
The chance of getting an egg is 69.3% (has a value of 70). The old man in the Day Care will say: "The two seem to get along very well."

Same species, same ID numbers
The chance of getting an egg is 49.5% (has a value of 50). The old man in the Day Care will say: "The two seem to get along."

Different species, different ID numbers
The chance of getting an egg is 49.5% (has a value of 50). The old man in the Day Care will say: "The two seem to get along."

Different species, same ID numbers
The chance of getting an egg is 19.8% (has a value of 20). The old man in the Day Care will say: "The two don't seem to like each other."

Different egg groups, same genders or Undiscovered group
The Pokémon are incompatible and therefore will not make an egg. The old man in the Day Care will say: "The two prefer to play with other Pokémon than each other."

Source and for more info:
http://bulbapedia.bulbagarden.net/wiki/Breeding

Answer 2

First they need to be breedable, which means they need to be in the same egg group and opposite genders, or one of the Pokemon is Ditto. (Note that legendaries can't breed at all.)

Then it's based on the OT (Original Trainer) of the Pokemon, and what species they are. Different species get on less well, and Pokemon from different OTs get on better. The exact criteria is as follows:

1. Different species, same OT: the Pokemon will not like each other much.
2. Different species, different OT or same species, same OT: the Pokemon will seem to get along.
3. Same species, different OT: the Pokemon will get along very well.