Insecuries? It wouldn't even cross Dazai's mind that you had them. You were perfect in his eyes. Everything that was yours was perfect for Osamu. So when he saw you standing in front of the mirror of your shared bedroom he approached you, wrapping his arms around your waist from behing, resting his shin on your shoulder, asking what's wrong. When he received as a response that you felt insecure, he couldn't help but laugh at that. The idea of you, thinking lowly about something he love and adored, maybe even obsessed about was almost amusing, so he naturally had to tease you about it, without exaggerating too much obviously.

"...what is it? Weight? Your thighs?...ah! I know, your chest size?~"