Why Do We See Ourselves As Morally Good?

It could be because it's beneficial for well-being and cooperation, but it could just be how we think about everything. We have a tendency to reflect on a thing's positive traits; this is called psychological essentialism. 

When describing the essence of a table, we say it has four legs, and a surface to eat on—the traits of a good table. We don’t describe a broken table. Our notion of the true self may be in line with this essentialist thinking.