Dazai lost everything he valued the moment he obtained it. For that reason, he was hesitant to be around you when he started feeling more comfortable with you. You made him a bit happier in his dull life and it scared him. He didn’t want to leave you, but he didn’t want you to get killed. So he decided he would have to be around you every moment that he could to ensure you weren’t in danger. His reasoning and actions were totally irrational. He was being too overprotective.