In recent history it's largely due to the Sykes-Picot agreement. The Ottoman Empire fell at a time when nationalism was arising and empires were going out of favor. In that power vacuum, inaccurate lines were drawn in the sand by colonial powers with little stake in life in the region. Since then every nation carved out of the area has been fighting over anything and everything because historically the area was ruled by large empires rather than the nation-states that were arbitrarily drawn there now. The history of the area is so old and intertwined that every country there has a claim of something on the other side of their borders within spitting distance. Western Europe had similar issues when the Western Roman Empire collapsed.