Maybe you give the wrong vibe, that you are not interested. Are you asking about them, are you also putting an effort to learn/know more about the person ?

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I usually loose interest (or I'm seeing it as a sign of disinterest) when a girl doesn't do any effort to know/learn about me. I will approach, i will start learning about the other person... but if it seems is one-sided I don't move further because ... there are some girls that don't just say "nope, i'm not interested / i have a boyfriend" they are also saying something like, "with you? no way", "uhhhh, no" or "eeeeew, no, i'm not interested", something that really affects anyone self-confidence. I dealt with all kind of girls and it isn't worthy to get a rude comment just because you show interest, this is why I want to see she is also putting some effort.

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This is me, if the girl doesn't put any effort I take it as a sign of disinterest and don't move forward.