The "lone wolf" shield he keeps using I can understand why it gets super annoying, but from his perspective he feels like he doesn't really deserve friends because he can't consistently keep up or make the effort to engage with them. He feels like the people who keep trying to be his friends are making a mistake, because he doesn't really deserve it and it takes away time from their other friends who he feels care more about them then him.
Its also somewhat very selfish of the girl to keep imposing the idea, no friends = miserable therefore I have to save him from it. Even if she says its not out of pity, it frankly just pity. Not everyone needs to be involved in other people's business, and is super selfish to weigh people's happiness on their own world views.