>I've tried introducing myself as "I'm a human", you'd think everyone would agree with this label but it gets unenthusiastic reactions. The followup "You're human too, right?" is even worse.
> I don't really understand why they react the way they do - it seems like a culture issue.
I'll dissect it with precision in order to assist you in escaping this particular social and mental trap.
Human social communication absolutely depends on fundamental shared assumptions.
Examples:
Murder is fundamentally bad
Literacy is fundamentally good
Even more fundamental than those:
Everyone literally has a head on their shoulders
Virtually everyone has a face on their head
Everyone is human
Whatever else your or anyone's unique thoughts or self-categorizations, those assumptions must exist or people won't want to engage with you.
Trying to bring them up as a point of discussion will place you in the range of "too weird" to "too crazy" to speak with.
In your specific case, I'd personally take the opener as a type of conversational manipulation. On top of being highly weird, because its an absolute assumption. Parallel to the type of thing that door to door salesmen say.
That is, they pose a question that they know 99% of people will feel compelled to agree with in order to force further conversation. That's you, with this opener.
Personally, I love not answering such questions from door to door sales people that are used to almost 100% compliance.
I give sales people the time of day who instead take a simple and sincere approach.
People are giving you a bad reaction because its a shared unspoken assumption, and because they are feeling manipulated. So, they feel manipulated by someone who is being very weird.
In contrast, introversion, age, and astrological signs are not shared assumptions. They are new information that therefore can be a socially acceptable substrate for further conversation.
Try talking to people about themselves, using simple questions and sincerity. It has a high success rate.