South China Sea may have a lot of oil and gas.
So usually the reasons are economic, they can also be strategic, and often they also have some element of domestic politics/nationalism.
For example the 'row' between Thailand and Cambodia over a small area containing a famous temple on the border was based on 1) Thai domestic politics - the Thaksinite side is friendlier to Cambodia and China, and when there was a yellow (conservative) government, they were quite hostile to Cambodia, perhaps precisely because they had sheltered Thaksin and supported him. 2) There is a significant conflict or issue between Cambodia and Thailand in the Gulf of Thailand, and there is oil and gas there, so some say that the temple issue is just an effort by either side to gain a 'bargaining chip'.
