Doing a favor is supposed to be an occasional event to help out another individual who is in a pinch. Doing favors all the time is simply being used. You need to do yourself a favor and address the matter with your boss. After all, this behavior is not going to stop until you tell him that enough is enough.
This does not mean that you snap back at him with a harsh "No!" the next time he asks you to take his car to the shop. Rather you should meet with him before he has a favor in mind and tell him that you are glad to help him out in a pinch, but the current level of errands is creating a real drain on your time and you need his help to put an end to it. Then be quiet.
His reaction is going to determine your next action. If he doesn't get the message, you can send him a more direct one the next time he asks you to take his parakeet to the vet. Simply tell him that you cannot do it because you have other plans. It's okay to do occasional favors for your boss, but this constant "favoritism" is too much.
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