Sometimes I feel it too hard to understand someone

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Recently, I feel quite upset. One of my friend promised to see World Expo in Shanghai with me on the coming Dragon Boat Festival, but he suddenly ignored me without any reason, i called him ,he didnot answer on purpose. I sent message to him asking why he treated me like that, he just said one word"sorry", and then vanished again. There were no quarrel or mistakes between us at all,i so cherish our friendship .now I get so confused and do not know what to do, break with him or just wait for his explanation,which may never come.Friends, has the same case happended to you? if you were me, what will you do?