Often the tone of conversations seem to carry that message (caring about anyone else means less care available for their brother). But in this case, I think "not seen as a threat to their relationship" comes from the relationship being seen as sexual (and monogamous). Or committed to brother-focused celibacy, I guess. It's only Ruby, and briefly Amy, who actually resulted in any conflict between Sam and Dean. And as an aside, I've become thoroughly sick of people who call for less conflict between them (even though there's been less of that cry since S7). A bit of conflict that gets resolved makes ~sparks~ and handholding or whatever you're into more likely.
Re: Extended TV Guide spoilers
Often the tone of conversations seem to carry that message (caring about anyone else means less care available for their brother). But in this case, I think "not seen as a threat to their relationship" comes from the relationship being seen as sexual (and monogamous). Or committed to brother-focused celibacy, I guess. It's only Ruby, and briefly Amy, who actually resulted in any conflict between Sam and Dean. And as an aside, I've become thoroughly sick of people who call for less conflict between them (even though there's been less of that cry since S7). A bit of conflict that gets resolved makes ~sparks~ and handholding or whatever you're into more likely.