Lots of kids feel mad, sad, and scared all together. They know that they didn't make their friend move away, and that their goldfish just got old and died, but they still have a part of themselves that's trying to find an answer. And, part of trying to find an answer is to blame themselves. But that's really not an answer at all. Kids don't make these things happen.
Sad things are just part of life.
But, happy things are, too. Sometimes they feel confused because they have all those feelings going on inside at once, so they're not sure what to do. Lots of times, mad comes out the easiest, so they get crabby and unpleasant. If that happens, they need to let the sad that's underneath come out. Then they're not the kind of person you need to be around right then because they don't understand that it's OK to cry.