In the previous post about mental maturity, an Idealist responded with something like "it's 'we' not 'me'".
While reading John Maxwell's second chapter of an upcoming book, he speaks about maturity
Maturity is the ability to see and act on behalf of others. Immature people don't see things from someone else's point of view. They rarely concern themselves with what's best for others. In many ways, they act like small children.
I think he expresses the Idealist's mental maturity quite well. It's not just being empathic. It's being so empathic that the other is more important.