Why did he pretend to be a good friend but wasnt ?

i really like a guy ive known 3 years. he knows this but said if we dated it would ruin our friendship. when we're around others he blanks me but alone he talks. he used to be very caring but has become abrupt and cocky. he puts it down to some trauma he had which i helped him through. aside liking him ive been a very good friend to him and helped him with money and work but he doesnt appreciate this and spends more time on the people who dont care. also he says he chats to other girls and know it causes a reaction-why does he do this. ive gone from knowing him very well to not at all. he lies aswell and this is wrong but ive seen his emails as he asked me to fix his cv and hes been lying about loads which is upsetting i was wrong for looking but glad i did why has he changed so much ? also why has he made me feel inadequate and like a stalker ? he was happy to chat loads before but has since finished his use of me :(