What is wrong with him?

first he used to flrit with me and i guess he was into me he asked me to the movies and i couldnt go . After that he still flirted with me sort of but right in front of me he sat with my friend and messed wiht her a lott and slowly he completely stopped giving me any sort of attention. Yesterday out of nowherehe was all over me flirting touching making dirty jokes at me talking and it was fun. today however he sat by me for a while and left then i saw him in the hall and i was walking next to this one girl he doesnt like at all. He completely ignores me and high fives her going all HEYYY (HERNAME) i was like okay wtf.... and that girl im not trying to soudn shallow or mean but shes like not the bestlookign girl out there and to top it off she has a crap load of attitude and i KNOW he doesnt think shes like hot