Do the same favor three times and it stops being a favor. It becomes a job description.

Repetition turns generosity into expectation, quietly, without anybody deciding to be unfair. By the third time nobody is asking, they are assuming, and the thanks fade because the act now looks like a feature of you. The shift happens in their head rather than through anything you did wrong.

Notice what you have done three times lately without being thanked. Skip it once, or say plainly that you cannot manage it this time. The reaction tells you quickly whether it was still being received as a favor.