The Happiness bar is always filled by 5 every 15 minutes, and is essential for many things in Torn City. You could say that your Happiness bar is a modifier bar. You want it to be high, and for good reason. Not only does it maximize your training, it also helps you do crimes, and bust, along with many other possible benefits.
Everyone starts out with a shack with 100 happiness. It is free with no tax. You can increase the size of this bar by buying a new house, but every time you do, your Happiness goes back down to 0 and your tax amount increases. There are very rare candies you can eat to boost your happiness, or you can use energy with certain items to increase it as well.
Not only this, but it is also possible to raise your happiness stat with drugs that can vary to doubling your happiness to simply increasing it by a limited amount. (However, overdoses of drugs can lead to hospitalizing yourself, and taking the drug too often can lead to an addiction which can be kicked in the Switzerland rehab centre for 250k).