I think this expression is so true. When you are missing someone you focus only on the good qualities. You don't think about what the other person does that annoys you, or parts of their body or behavior that you find ugly. You are not remembering any past slights or the faults in their personality. You only focus on the positives.
When you see a person after a long absence its like they have been refurbished in your mind. They are their best selves all over again.