Snapedom

The intensity of his gaze made her blush....

"--He fancies you, James Potter fancies you!" The words seemed wrenched from him against his will. "And he's not... everyone thinks... big Quidditch hero--" Snape's bitterness and dislike were rendering him incoherent, and Lily's eyebrows were traveling farther and farther up her forehead.

"I know James Potter is an arrogant toerag," she said, cutting across Snape. "I don't need you to tell me that. But Mulciber's and Avery's idea of humor is just evil. Evil, Sev."
...
The moment she had insulted James Potter, his whole body had relaxed....

Don't those climbing eyebrows rather look as though Lily had just realized that some (maybe she thinks, all) of Sev's dislike of James is jealousy?

And her immediate response was to reassure Sev, NO, she doesn't like his rival! But back to the real point of contention--

Lily doesn't grab Sev and snog him, no. (Drifts off into fanfic....)

But if she HAS just realized that somebody besides James "fancies her," she's not cutting him off cold. If anything, her hastily diverting the discussion back to Mulciber and Avery suggests she's trying to buy time. She's not ready to deal with his feelings; she's not sure of her own.

But she certainly doesn't.... Um. Ouch. The school dance when I was 14, someone told a retarded boy that I liked him. And he believed hir, and was flattered, and lumbered up and asked me to dance....

Needless to say, I am (and was then) ashamed of my response, but I don't think, realistically, most teens would have been more graceful or less cruel. Or less, um, quick to get rid of the embarassment.

So trust me when I say, reassuring Severus (I hate the boy you're imagining as a rival) and hastily changing the subject, is NOT the response of someone embarassed that a clear inferior presumies to take a sexual interest.

Or even the response of someone who simply wants to cut this off before it starts. (That would be more: "My opinion of James Potter is none of your business, and it's none of your business who fancies me, either)

Moreover, she had "blushed" earlier, not flushed in embarassment or anger, at "the intensity of his gaze." I.e. Sev's saying "I won't let you" may get her dander up, but she LIKES being the focus of his intense attention.

Finally, I know everyone's different and all, but... at fifteen I wasn't that quick on picking up nonverbal cues of interest. (Indeed, at twenty I wasn't always.) The only way I'd be that quick to spot signs of, um, more romantic interest from one of my best male buddies was, er, if I was a bit looking for them. On some level.

*

So. Finally. To get to the point you were actually addressing: at fifteen, when you had crushes, did you find it made your JUDGMENT better about the subject? Paying more attention to hir, yes; paying inordinate and obsessive amounts of attention to hir, sometimes; but--clearly assessing the loved one's strengths and weaknesses? (Breaks into helpless laughter.)

If I remember correctly [channels Albus], I think, at that age, I was rather more inclined to whitewash the beloved. Certainly I rather closed my eyes, if I could--and somehow it usually seemed I could--to possible character defects. (However glaring apparent those defects later appeared.)

Now, think back to Harry's reassurance to his younger son in the Epilogue. Harry didn't tell ASP that his Slytherin namesake was the cleverest, most loyal, most devious and determined man he'd ever known, though all would have been true.

Harry said Severus was the bravest.

The ultimate, the only, Gryff accolade.

And I really wouldn't call on a teen to distinguish between stupid risk-taking for thrills and the principled braving of death for a purpose. James was capable of the first, and probably of the second; Severus was unabashedly capable only of the second.

So if Lily refuses to notice that her best friend is, um, timid in comparison to her housemates ... maybe it's because she doesn't want to notice it. Because that would diminish him in her eyes. As he's trying his best to hide that lack of relish in risk-taking from her, for the same reason.

For the same reason.