<human>: * The seat is there for one reason, to sit on.  

* The number one criteria that the seat should meet before being sat on is cleanliness.  

* It is much less likely that the seat will get dirty if it is left up when not in use, because, let's be honest, not everyone takes the time to lift it before using the toilet, and not everyone has perfect aim all of the time.  Sure, some people clean it up if they miss a little, but not everyone.

* This goes for both girls and guys.  Both are capable of dripping on the seat and neglecting to clean it up, and both are capable of peeing with the seat up (standing or squatting).

* If you're worried about the aesthetics of the toilet, put the cover down.  This will force someone to lift it before using the toilet, and if they are already lifting the cover, there is a better chance they will lift the seat, too.

Condense the text into a few key points
<bot>: If the seat is left up when not in use, there is a much higher chance of it being clean when someone wants to use it.