[Armansky:] "You're not a person who encourages friendship," he said. (2.24)
Armansky's not telling Salander anything she doesn't already know. She hasn't had too many good experiences with people, though some major details of her troubled past don't emerge until the final installment of the trilogy.
He relaxed and the anxious knot in his stomach had eased. She had that effect on him. She always had had. And he knew that he had the same effect on her. (3.36)
Although sex is a major factor in Blomkvist and Berger's relationship, this quote gives us the real key to their friendship. They argue and bicker a lot, but they ultimately understand and enhance each other's personal and professional lives.
Around 7:00 he heard a loud meowing at the door. A reddish-brown cat slipped swiftly past him into the warmth. (8.112)
Blomkvist's friendship with Tjorven the cat isn't a huge factor in the plot, but it helps us see how friendly Blomkvist is, in contrast to Salander.
Like a faithful watchdog, she kept an eye on every doctor and nurse who walked in or out of the door. (9.54)
Holger Palmgren is one of Salander's friends. Palmgren is the guy who taught her to analyze "consequences" (11.168) before she acts.
[Martin's] lady friend joined them for dinner. Eva was a warm, sociable, and entertaining woman. Blomkvist found her extraordinarily attractive. (10.6)
This contributes to the general consensus that Martin is a nice guy. So when we learn that he's a serial kidnapper/rapist/killer, it gives us chills and makes us want to know more about Eva.
[Bjurman:] "If you're nice to me, I'll be nice to you. […] If you make trouble, I can put you away in an institution for the rest of your life. Would you like that? (11.172)
Bjurman doesn't know the meaning of the word "nice" and has no intention of being a friend to Salander like her old guardian Holger Palmgren. Too bad for Bjurman!
[Blomkvist:] "This is the way it is – Erika is my best friend. She and I have been together off and on for twenty years […]. […] But we've never been a couple and we never get in the way of each other's romances." (15.97)
Cecilia Vanger isn't much reassured by Blomkvist's description of his relationship with Berger. Most women Blomkvist meets aren't, including Salander. Why do you think this is?
She stared straight into the eyes of Mikael Blomkvist, felt panic race through her body, and took a step back. (18.4)
We just love this first moment when Salander and Blomkvist meet. We've been waiting for it for seventeen whole chapters!
Blomkvist had known her for five minutes and she was practically giggling with him. [Armansky] felt such a loathing for Blomkvist that he surprised himself. (18.79)
Poor Armansky – he just doesn't know what to do with his feelings for Salander. He's never met anybody like her. Yet, he views her as exotic, while Blomkvist sees her as a cool chick.
[Blomkvist:] "I understand that you don't want to discuss yourself with me, but someday you're going to have to decide whether you trust me or not. I want us to be friends, but I can't do it all by myself." (26.100)
For Blomkvist, friendship means intimacy, and intimacy means (for him) shared knowledge of each other's lives. He knows Salander is holding back because she doesn't fully trust him.
Damnit, he had treated her like a human being. (21.134)
Blomkvist likes Salander for who she is. But she's not sure he will if he knows her secrets and the horrors of her life. Right now, she doesn't feel very human.
[Harriet:] "It feels fantastic to tell the truth. So now you know." (26.121)
Harriet confesses everything to Blomkvist, even killing her father. Is this the start of a beautiful friendship?