It depends on the ditto and pignite.
If they are both from different original trainers, the day care man will tell you, "The two seem to get along." (or something similar)
This means that every 256 steps you take, theres a 50% chance of getting an egg
If they are from the same original trainer, the day care man will say, "The two don't seem to like each other that much." (or something similar)
This means that every 256 steps you take, theres a 20% chance of getting an egg
In newer games, theres also an item called the Oval Charm that increases the chance.
Source