To "fit in" with everyone else is an evolutionary survival mechanism, because standing out from the herd will make you an easy prey.
Cults, clubs, and fanatical fans are all extreme examples of the power of social proof at play. But there are many subtle variations of it. The brands you like, the fashions you follow, the music you listen to... the list goes on.
We always look to others for approval because being like others is SAFE. We act like others do because we don't want to seem flustered or out of place.
Even though the actors and writers hate the idea of using canned laughter, producers know it works and they keep using it. Even though the public knows it is 'canned' they soon forget and take their lead from the soundtrack as 'proof' that the show is funny.
Copy cat serial killers abound. Copy cat suicides are rife. In fact research has proven that when a major/ newsworthy suicide has occurred and has been publicised, suicide rates in that area increase by 1000%.
We learn about our own abilities and attitudes by comparing ourselves with other people and their opinions. Mostly, we seek to compare ourselves with someone whom we believe we should are reasonably similar, although in the absence of such a benchmark, we will use almost anyone.
We make fewer mistakes when we do what other people do. It is a powerful shortcut because it has been ingrained in us since we are children.
We all want to be recognised as individuals, but we don't want to be individual enough to be laughed at.
Social proof is especially powerful when the people we are comparing ourselves to or SIMILAR to us. We say we don't care what other think, but we only say that when others are OTHERS - and we care deeply what people like us think. A mother cares what other mothers think. Teenagers care what other teenagers think about them. Teenagers rebel against their parents but conform to their peers. (People even experience less physical pain if they are in the company of people pretending not to feel the 'appropriate level of pain'.)