The ponies, like anyone, have off days and are still learning about friendship and how to interact with other ponies! I mean, if they treated my badly I would definitely feel hurt, but I will forgive them if they decided to say sorry for whatever they had done!
I think they can sometimes be mean without realising, ie, EG2; they don't let Sunset Shimmer into the band until the very end, and also forget she is there (which is pretty hard on someone, even if its not explicitly being mean to them).