Whoever formulated the pop psychology glass half full/half empty theory really should have asked drinker about it.
Look, it’s simple. If you pour, say, a half-pint bottle of beer into an empty pint glass, then the glass is half full. If you start with a full pint and drink half, the glass is, quite clearly, half empty.
Obviously, then, whether you consider your glass to be half full or half empty reflects, not upon your state of mind, but on the condition of the glass at the outset.