How have they not plapped the Earth into dust yet?
It's easy!
Mikoto is desperate to keep Takase in her life now that The Acid Attack brought him back.
Takase feels he is not worthy to have Mikoto as more than a childhood friend because:
he 'let' their relationship drift apart, which means...
he wasn't there to even try to protect her from The Acid Attack, and thus...
he is disgusted with himself because his fetish for scarred childhood friends means she's only become more attractive to him.