Just like people, dogs have separation anxiety. As for what causes it, there are many. As a result, the dog and the owner can have quite the difficult experience. Overcoming the separation anxiety can be work, but it pays off for both the dog and the owner in the end because it is healthier and makes the overall environment more bearable.
However, you have to determine that what your dog is suffering from is really separation anxiety. Separation anxiety can mimic a number of other issues. Just because your dog may chew things, tear things up, and have accidents in your house when you're gone, that doesn't mean your dog has separation anxiety. These are non-anxiety issues that need to be addressed. Excessive barking does not indicate separation anxiety because barking can sometimes mean the dog is just bored and requires some dog obedience training. Sometimes the way to remedy this is by incorporating more exercise.