Don't kill the green horses!

Today I will tell you a story which was first said by one of my great university teachers. So, this is it:

"Once upon a time, there was a beautiful little princess who would tell her nanny that every time she looked her window she would see a herd of green horses. The nanny couldn't see the herd and neither no one else. Being worried, nanny told the princess's parents about the girl and firstly they thought she was just a child and hoped that by the time the girl would grow up she would stop seeing the herd. But the girl turned up into a gorgeous, intelligent and talented young lady continuing to see the green horses herd. So, the king and the queen decided that she should be seen by a doctor. Since that day, the princess started to rarely see the herd, until she stopped seeing it at all, but also she turned sadder and sadder. One day, her parents begun to ask themselves what is wrong with the girl, so they asked their daughter. Her answer was that she was sad because she couldn't see the green horses anymore. Therefore, the king and the queen decided that they preferred a daughter who could see green horses but who was happy, to a sad girl who yearned for her lost green horses herd. So, the princess begun to call for her herd and when it came out of the trees she became the happiest person in the world."

I'll let you figure out what it really is about. :)