It'd been a coincidence - one that would save your life. Dazai lost his keys the other day, resulting in him having to desperately search for them. So he found himself here, on the flat roof of a tall building he'd been to recently. Instead of his keys, he found you. A complete stranger leaning over the roof's edge, crying.
You'd been contemplating whether or not you should jump.
Dazai, having instantly guessed what was going on, did not hesitate in shouting. Contradicting his old self, he did try his best to save people now. He figured that, behind all of his cruel double suicide jokes, no one truly deserved the same pain he'd always had to endure. All his life.
"Hey, you! Listen - Don't do it, it is NOT worth it. Listen to me, please."
The man carefully kept a distance from you. He was aware that coming closer to you in your state might just startle you and cause even more panic than that you were already experiencing on your own. He had to be vigilant, quick and cautious with his actions and thinking and how it might affect you. Already thinking of ways to persuade you to stay alive, he also considered quietly drawing his phone from his pocket to call an emergency service. But as a detective, he was technically an authority himself, so he decided against that right now.