He agrees to a date and then starts negotiating it down a week later

Every time we set an end date he is enthusiastic about it, and then between a week and ten days later he starts making a case for why it should be shorter. Never a demand. Always reasonable, always with a good argument attached, usually something about work stress or a trip coming up.

I have given in twice and held firm twice. Holding firm made him sulk for a day and then he was fine and told me afterwards he was glad. Giving in made him deflate almost immediately.

So the pattern says hold firm, but I feel like I am being managed into a game I did not agree to play, and I am tired of it. Am I missing something obvious?