How can I get 100% happiness?

Previously, my city's population had a happiness of 100%. After some new building, their happiness dipped to about 90% (because there was no longer enough water or sewage treatment).

After fixing that (and creating more buildings) the happiness has risen to 99%. But, I cannot work out how to get that last 1%.

I have checked:

• fire stations
• police stations
• parks
• water
• power
• sewage
• traffic

It all seems to be OK. EA answers did not provide any new clues.

Currently, my city is at level 13.

If you want to get more happiness, you should built some population boost building like casino, I build many of it and the happiness never dropped below 100% although I remove all the park.

Maybe you should check waste management and factories and whether the dirtiness from power, sewage, waste management and factories touches the houses.