Another point that maybe didn't get emphasized enough is that studio monitors are generally designed for near field monitoring, as mentioned by BlueJays above. They are intended to be as accurate as possible when listened to in the sweet spot of an equilateral triangle from a very close position. They are often designed as mirror images of each other. The intent is that, since room reflections are difficult to control, you want to be so close that those reflections become insignificant and all you are hearing is only and exactly what's being recorded.
Most home speakers, OTOH, are designed to include the sound of the room in their character. They are intended to sound "warm" or "sweet" or "bright" or "clean" or some such. Using such speakers to mix music will often lead to surprising results when a mix that sounds "just right" on them is played on something else somewhere else.
