[@Euphemia]
It really all depends on preferences. 
like, if it is with more modern characters... modern day
but if fantasy characters, human or not, are involved... fantasy. 


me? I usually don't do strictly sci-fi stuff myself. 


as for motivation, it can be anything. 
Ending a war.
money. 
power. 
money AND power. 
someone simply looking to marry someone off. 
alliance purposes outside of a war or the like. 
Anything.