"Why are Christians always fighting?"

If they do, they may be Christians but not disciples of Jesus Christ. Otherwise they would love one another (John 13:34-35), or love even their enemies (Matt. 5:44) as that is what Christ commanded. But if they cannot obey His command then they do not really love Jesus Christ and the love of God is not in them (John 14:15-24).

Even as Paul said that those of Israel are not really Israel (Rom. 9:6), and Jesus Christ's word to the churches in Asia that them which say they are Jews, and are not, but are the synagogue of Satan (Rev. 2:9 & 3:9); Not all Christians are true disciples or followers of Christ, not all are of God. In the church of God, there are wheat and there are tares. God knows who are His but you will know them by their fruits (Matt. 7:16-20).