Why are thank-you notes so difficult to write?

So, Mom's been pressuring me to write my thank-you notes for my birthday, and she asked me what's so hard about it. I said, "Dry mouth, shaking hands, blurry vision, and nausea." Lis agrees since that's what happens to her, too, when she has to write a thank-you note, and we also both agree that the requirement to write thank you notes is one of the reasons why we both hate getting presents so much.

What is it about thank you notes that does that?