I think Asahi has never experienced special birthdays

He has a very low opinion of himself. His self-worth is always put into question. He had a difficult childhood. Years of holding bitter thoughts and experiences would make him disregard the importance of his own birthday and that treatment would seep to his attitude towards the birthdays of others. I'm not defending what he failed to do for Masato's birthday which is the bare minimum. I'm just trying to understand. Personally, I also don't feel like celebrating my birthdays owing to my low self-esteem.