I've also experienced This type of Loneliness - because I haven't been as slim as the rest of my class - so that was enough for them to reject, totally ignore or bully me ( if you've ever experienced such treatment at school for the same or any other reason - You know how cruel school kids can be for their peers... I don't wish it to Anyone... bad memories... )

You don't look like like them - you're fat, skinny, short, have the hair color they don't like, wear glasses - or Anything else - Kids really can find hundreds of 'stupid reasons' like this not to make friends with you...

They can even avoid or bully you when you are new, poorer than them, or learn better than them & have better grades - the possibilities they find a Really Countless...

Even worse if you are shy, easily scared or easy to annoy or to make cry - then some bullies will most probably find you - because you may be a 'perfect victim' for them