it can be a lot of causes, including traumas and mental health issues but a lot of times it can be their parent and what their parents transmit to the kid, as soon as a baby starts developing he can receive emotion on his mom's belly and if he feels rejection he will start growing up with issues. Their enviroment can also play a big part on it for example if a kid lives in a bad neighborhood he will have a hard time knowing whats right and wrong based on what he sees and other's behaviour. Another cause can be what a kid hears, when kids are developing whatever they hear it will be engrave on their minds, which is what causes a lot of times for kids to grow with an unstable mental health.