What's the most efficient way to raise the happiness values in my settlement?

Apart from having enough food and water, a strong defense (higher than food + water total to minimize attacks) and a general absence of attacks on your community, the happiness is mainly controlled by decorations and luxury.

Adding televisions, rugs and non-essential furniture (anything other than beds) to your settlement will increase the happiness of people living there. Basically, if you try to furnish their homes with care, they appreciate it. Who would have thought?

In one of those endless "How to foster your settlements" guides I read that apparently, the most efficient way to raise happiness in terms of resource/performance is packing any and every vertical surface with paintings. They require only wood to build which should be in overabundance and don't take up space on the ground for essential things.

According to this Reddit thread (which was linked in a comment by Hentold, but seems to have been ignored otherwise... I found that thread and this question when trying to figure this out), happiness has a number of factors. Some of the other answers on this question do not have the correct factors, though.

First off, settlement happiness is an average value computed from each individual settler's happiness. Humans can have a happiness between 0 and 80, while non-humans have their happiness set to 50.

Humans' happiness is a factor of:

• A place to sleep (under a roof)
• 1 food to eat
• 1 water to drink
• A safety value greater than the number of settlers

Not meeting these basic requirements causes very low happiness for an individual settler.

Once all the settlers' individual happiness numbers are computed, a bonus is added to the total before dividing to get the average. This bonus can be influenced by:

• The junkyard dog (or other pets, gorillas and cats with the DLC)
• Stores (+8 to +40, depending on type and size, Large Bar being the best)

Once the average is computed, modifiers are applied. The influences here are settlement quests and settler deaths. This modifier decays towards 0 at a rate of 20% per day. If you bork up a quest or someone dies, happiness will take a dive but then eventually bounce back.

The computed happiness value doesn't take effect immediately, however. changes between current and computed happiness take place over the course of several days. So, if you mass build a bunch of things that ought to impact happiness in a settlement, it will take time for it to reach the new equilibrium.

Keep all of the resources in the green, and build them some non-utilitarian stuff (chairs, pictures, etc) and the happiness should go up