Why can’t my sister accept the fact that a guy may genuinely like me for me?

I developed a crush on this guy I work with and from the moment I did I told my sister all about it. She’s like my best friend and I trust and love her that much. At first, it was a casual crush. Then we started to engage in conversation. We have started off learning little things about each other. We both light up with smiles when we see each other and always give each other a hug. Our conversations are natural. I even got him to start watching a show I recommend him to watch and now we have something more to talk about when we see each other. Even one of my other co workers, when I told her that he was the guy I liked and pointed him out to her, she even noticed the way he interacted with me and said “You can tell he has a crush on you too.” I will sometimes joke to sister and say “I saw my man today.” “Me and my man talked about etc etc today.” And she’ll laugh and call me delusional and I’ll laugh too because I know he isn’t really my man. Her saying that isn’t what got me down. What got me down was when I mentioned to her that I think he likes me too and how another co worker said he has a crush on me as well, she laughed and still told me it was all in my head and right after saying that she mentioned how a guy she works with, is always on her “tip” and how he basically told her he can teach her things in bed. It sounds like he just wants sex and my sister says that’s all she wants as well which is fine but why is it so hard for her to believe that a guy actually likes me for me and not sex but if she says a guy is all on her “tip” off back, it’s not in her head and it’s true.

Why can’t my sister accept the fact that a guy may genuinely like me for me?
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