You can't know that the person is interested or if he or she was just being friendly at a social gathering. How can you tell if maybe there is something more there? Of course, if you had only met once, that is not enough always to kindle a lifelong romance, and so it would behoove you to maybe make contact, call the person up, send a text message, or offer to hang out again and see each other another time.
Now, you also want to make sure that you do not come on too strong. Often, even if the first encounter was a success, coming on too strong too fast can send the person running in the other direction. Moreover, you want to see if that person is interested, which means you have to give them space to show their interest.
If you are the one always text messaging them first, or arranging to meet up first and they are either giving you excuses for why they cannot or supplying closed ended answers, then you need to assume that maybe they are not that interested. In that case, you have to give giant gaps of time in between contacting them.
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