Detecting if someone likes you or not is similar to detecting if someone is lying or not. "Lie detectors," human or machine, are actually anxiety detectors since the physical signs of anxiety are associated with lying. While a person can have anxiety about lying, he can also have anxiety about being accused of dishonesty—or he can simply be an anxious person. Sociopaths rarely show signs of anxiety when lying, so these detection methods can be more than useless, but lead to false negatives (i.e, thinking the person is telling the truth when they are in fact lying). As with lying, the physical and detectable signs associated with liking are often better indicators of something else.
It is important to point out that "interest" is very different from "liking," and I am focusing on the latter here. A wink and a smile by a stranger in a bar might be a strong sign of romantic or sexual interest, but these kinds of signs are often similar to taking interest in an object. Liking someone requires a deeper emotional involvement in the person that strengthens with exposure.
Consider the person's motivations. Some people are paid to be nice to others—basically anyone in the service industry. I am reminded of the guy who goes to a strip club and insists, "oh yeah, she wants me," referring to the dancer he just paid $20 for a 2-minute private dance. In many social situations, we are more like actors playing a role than humans communicating our authentic feelings to one another.
Consider the situation. A part of social intelligence is the ability to interpret the social interaction while incorporating situational factors. Is the other person distracted? Did they have a bad day? Are they in a rush? Do they have to pee? Don't assume that a person's behavior toward you has to do with them or you—there is a very important third player in the game of social interactions, and that is the situation.
Friendly people don't necessarily like you. There are friendly people who have warm and inviting personalities, strong social skills, and enough charisma and charm to convince people that they are their new "bestest" friend. The chances are, their interest in you is limited.
Unfriendly people don't necessarily dislike you. There are unfriendly people who have cold and standoffish personalities, poor social skills, and lack the charisma to motivate people to not fall asleep while interacting with them. These dispositional traits have nothing to do with how the person might feel about you.
Assume people like you. Unless you are a real jerk, the chances are people like you. To assume otherwise is to have an unrealistic and unhealthy self-image and perhaps an unwarranted pessimistic view of humanity.
The most accurate way of knowing if someone likes you or not is to ask them. But, of course, this can be awkward and is not recommended in most situations. If you like someone and are interested in building a relationship with that person, make an effort to reach out to that person on one or more occasions. A good sign that the person likes you is not just a response from that person, but a response that is conducive to further contact, such as questions or suggesting future plans together (e.g., "we should get together and eat caramels"). If you are finding that the relationship you are attempting to build is too one-sided, whether it is due to the other person not liking you or something else completely different, you may want to back off. There are many people in the world who would be honored to have a new friend, so your efforts might be better spent focusing on those who will reciprocate your friendship.
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