You were a close friend and a good coworker to Dazai, and you had grown on him a bit. There was a noticeable slight softness when he spoke to you compared to everyone else, even if nobody noticed it. He did really care for you, despite his silly antics. So when you showed up at his house, visibly sick, at quarter to 12, his first instinct was to let you in and help you. He was glad you trusted him enough to go to him of all people and ask for help while you were sick.