People give you a (often vanishingly small) window of opportunity to prove you are worth their acceptance/friendship. Once this window of opportunity closes it’s pretty useless to even engage them. The window’s size varies from person to person. Some people will notice your awkwardness or uncool cloths and immediately disengage communication. Others will probe you a little to see if you share some of their interests and back away if you don’t. Therapists will tell you that in social situations you shouldn’t be so self-conscious because people aren’t thinking about you. In a way this is true—they aren’t having any novel thoughts about you. However, every interaction with you they have are informed by the split second they made up their mind about you in the moment where that window of opportunity closed and they decided to reject you.
An example: you are a junior in high school and it’s your first day of Spanish class. The person behind you is being friendly to you because you both beaten Skyrim and unlocked most of the Steam achievements. These conversations go on a couple of days. However, a few days in you are having a stomach ache and ask the teacher if you can use the restroom. She says no and then you have somewhat of an outburst because it should make sense to her that crapping in the toilet rather than on the floor is the better of the two options. After this outburst your potential friend only gives you one or two word answers to your attempts at socializing. Your window of opportunity has closed.
It goes without saying that in the window of opportunity a lot of people are just looking for a reason to reject you. This is because while some people value common interest and virtue, most value adherence to social norms and social status above all else.
Those in power strongly disapprove of the reality that people give you a window of opportunity to get to know them (and any efforts to do so after this window has closed are futile). This is because a lot of what keeps a person in line in social situations is the hope that, by behaving, he can earn the respect and friendship of one or more of his peers. When this hope is taken out of the equation, there is no real incentive to conform.
I can remember the end of high school at a school picnic, I knew I had no chance at being accepted by any of the guys. So I actually chased butterflies.
Also, after giving up on caring (after eclipsing people’s window of opportunity), these people will jerk the hope chain. They will claim that they would have accepted you if you would have just acted according to their wishes just a little bit longer. This is to try to get you to feel remorseful because that there was some hope of them engaging you positively left (when there really wasn’t).