You and two others are tossing a ball around. When it comes to you, tap someone to throw it on. That is all, just play for a moment.
The other two stopped throwing to you, and something noticed at once: a small drop, maybe a flash of "hey, why am I being left out?" See how little it took. A pointless ball game, two people you will never meet, nothing at stake. And it was scripted: they were always going to freeze you out.
That instant sting is the need to belong sounding its alarm. In hundreds of studies using exactly this task, called Cyberball, being left out reliably lowers mood and rattles people's sense of belonging within minutes, even when they are told the other players are only a computer (Williams, 2007). The need runs so deep that the mind flinches at exclusion that means nothing at all.