Why are people so mean ?

ok sooo this person named mandy is just sooo over ratedly mean ... ehhh she is such a 9999999 i cnat take it she akways things in a mean way and trys t do pranks to me that she thinks are funnie even thought they r no where close to funnie more like offending ,,, should i continue being her friend or ditch er ? y do u think she is acting lik this ?