新目标八年级下Unit2 Section B 3a

Dear Mary,
I have a problem,and I need your help.I always thought I was popular at school.But I just found out that my friends were planning a birthday party for my best friend,and they didn't invite me.Everyone else in my class was invited except me,and I don't know why.I can't think what I did wrong.I'm very upset and don't know what to do.What do you think?Can you help me?
Yours,
Lonely Kid