do you initially assess whether your gf/bf's friend is a competition?

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i just want to know whether guys or girls instantly and initially investigate and assess whether a newly-introduced friend/classmate of his/her girlfriend or boyfriend is a competition. whether that new acquaintance has a hidden agenda of being friends with your gf/bf just to gain their trust and possibly steal your girl/guy? is this like an insecurity issue or you're just naturally protective of what is yours?