Answer:
Jesus' grief was caused by the fact that he knew he was entering Jerusalem to fulfill his destiny as the Messiah and that it would lead to his death. Jesus was deeply aware of the suffering and death that awaited him and the sorrow that would be brought upon his disciples, family, and followers. He was also aware of the pain and suffering that would be caused by the Roman occupation of Jerusalem. This weight of knowledge and the sorrow he felt for these things made his triumphal entry less triumphant.