To fix this, build more Cultural Buildings, keep your roads updated, and keep a low population. If you have a lot of spare population, sell some regular Residential Buildings, keeping Event Buildings. Some Event Buildings from Events provide a huge lot of Happiness, so it's not a bad idea to get one or two. And even some Great Buildings, such as the Hagia Sophia, Frauenkirche of Dresden, and the Alcatraz will help with that as well.