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They see everything in a positive way. They overlook the negative things. To them, the glass would be half full rather than half empty.

When looking through rose-colored glasses a person sees only the happy things in life - everything looks rosy. They choose to happily ignore the bad sides of life, and take the optomists point of view.

they are looking at things through a "happy glass". That they are not necessarily ignoring that bad stuff has happened, but choosing to not see it, and only seeing what they want to see...

It means they view the most positive side of life and the most positive side of things.