This is a great question and one that is important to our world today. For example, Israel and Palestine are fighting over land right now and this conflict can be disastrous to many. China is also disputing Japan and the Philippines over two islands as well. So, this question is very alive and well today.
With that said, there are many reasons why nations dispute over land. Perhaps the greatest reason why nations dispute land is due to natural resources. What makes land very valuable is what is on or under the land. For example, is there is gas, gold, oil, or any number of other commodities, then that land is very desirable. This is the case with China, Japan and the Philippines. There are debating the deposit of natural gases in the disputed area. Another reason of dispute is access to water, which make the land more fertile. In short, there is usually something about the land that both nations want.