There's a slight difference between Sella complaining to Shirou that he does too much and now Shirou mentioning it's his turn this time to cook food. I think the normal situation would be the maids having solid turns to make food and the masters of the house only preparing dinner irregularly whenever it happens to strike their fancy (if they have such hobbies). Well, of course we are talking about Shirou here, so I reckon he might actually feel bad for the maids (even though they are doing their fricking job) and thinking he must let them rest sometimes. That would be just like Shirou who always believes he knows what's best for everyone.