You and Dazai made a bet, playing pool (billiards) with each shot you miss you will take one cloth off in this cold weather. It was the end of November and the table was at open area too

Dazai was playing so bad before the bets, it was his part of plan so you'd agree to bet, and you fell for it, now here you are... Not even getting a chance, he kept pulling moves that makes your balls in an impossible position to hit to hole, which is why you're upper clothes are off, jacket, sweaters.

suddenly Dazai pushes you to table, holding cue, aiming to the last ball, 8 ball. hitting it perfectly as it goes in hole, meaning he won. He looked at you smirking before his hands went on your chest and slides to hips, and then stands up properly

"I gotta say this was the best game I've ever experienced~"