Why are girls so mean?

Throughout my life there have been quite a few girls out there who hate me and are so mean to me. most of whom I don't really know or have met once but briefly. I don't think that I deserve to be treated so harshly by these girls. I feel that I am a nice person and I treat everyone with respect. I don't feel this is very fair. Can anyone give me some answers? Why are girls so mean?