Now, this movie has been remade and reimagined about a dozen times since it first premiered. I have only ever seen the original but I will be discussing the general premise here. First, from an economic point of view, this is an ingenious idea for a horror movie. It all relies on the actors rather than special effects. People you know one day are seemingly replaced and something is just kind of...off...about them.
The original movie was apparently meant as an allegory about the insidious spread of communism (they look just like us and live among us, how can we tell if they are looking brainwash our children?). After this recent election, I feel like the parable is even more relevant. People you thought were decent or kind turn out to be aligned with a malignant force, undermining your faith in society as a whole.
As a person who suffers from low self-esteem, I can totally relate to the feeling that the entire world is against you. Probably, we've all had days where we turn to friends for help and they somehow add to the problem. As far as real fears go, feeling alone in a crowd is one of my worst. This sci-fi horror takes that fear and amplifies it to paranoia levels.