Pokémon Breeding Guide - Chapter 1

Compatibility

Not all Pokemon like each other, so you'll have to do a compatibility test by talking to your Pokemon in the backyard. You can pretty well tell if you're going to get an Egg or not by watching the responses.

If it appears to shows an interest in the other Pokemon, and is friendly, you have a good chance of getting an Egg. If it seems restless, and/or apathetic, your chances aren't that good.