Why? Life is a mix of things: death, sorrow, happiness, loneliness, fun, friends, bitches, ignorance, sex, romance, elves and such.
You might be writing/reading a historical novel, or a romance, or a spy thriller but personally I think that, regardless of the genre, fiction should be more realistic in an all inclusive type of way.
I become quickly bored with monothematic books. Fiftyshades of gray managed to make S&M boring to me because the plot was so shaky. (Also, I’m not into S&M at all, but that’s personal. Pain is NOT SEXY to me.) Sappy romances bore me because I already know what’s gonna happen. Pure action novels bore me because they have no character depth.