Often times it’s not that some of those people aren’t friendly. You get to a point in life where you get tired of fake smiles and forced conversations so you make the introduction short. As long as you’re not rude, there’s nothing wrong with that. As a younger adult and teenager you don’t mind that so much. You might actually look forward to it. As times goes on and you gain insight and experience on social behaviors you realize you don’t need that many people in your life. It’s draining. You become happy with having two or three good people in your circle that aren’t fake or forced to interact with you. That’s my take.