I think she might not have wanted to accept it because she was friends with him and didn't really want anything more than that. So, I think she may have tried to ignore the fact that Snape had feelings for her and try to carry on as normal because she didn't want to hurt his feelings.

I think she realized it and probably reciprocated those feelings too, up until the point he called her a mudblood after she defended him in front of James. That must have hurt her to her core, beyond any forgivable point especially since he was involved with a not so favorable crowd of students and was quite into the dark arts. After that I think is when she closed herself off from Snape and later fell for James.

She's supposedly referred to as smart by many people, so I think she might have noticed.

I reckon she just thought they were like, best friends.

I agree with JustMe7 & shieldmaiden