Was it the sex that changed his value?

I’ve seen some comments saying he was some kind of science experiment (cuz of his arms and neck) and his memory’s had been erased as well as his feelings? I think it maybe gave him a sense of pleasure again

It was probably the chapter where Teach convinced him to finally get sleep for the first time in his life that changed him. Guwon told him his secret that he doesn’t have any memories or knowledge of who he is before going to sleep and felt like Teach still accepted him. He got really into kissing Teach after that.