mulderitsme:

“Dana?’ [asked Garson]. She nodded to show him she was listening. ‘Why does he call you Scully all the time? I mean, you do have a first name.’ ‘Because he can,’ she answered simply, without sarcasm, and didn’t bother to explain. Just as it would be hard to explain why Mulder was, without question, the best friend she had. It was more than just being partners, being able to rely on each other when one of them was in danger, or when one of them needed a boost when a case seemed to be going bad; and it was more than simply their contrasting styles, which, perversely to some, complemented each other perfectly. What it was, she sometimes thought, was an indefinable instinct, a silent signal that let her know that whatever else changed, whatever else happened, Mulder would always be there when he had to be. One way or another.”
―The X-Files: Whirlwind by Charles Grant