Happiness is a mechanic exclusive to My Singing Monsters which affects the Currency production rates of Monsters.
Mechanics[]
Most Monsters have a happiness meter that is influenced by their Likes, which can be other Monsters or Decorations. For every unique liked item that is placed near the Monster, the Monster's happiness is increased by 25% (multiple copies of same item will only increase happiness once), thus four items are required to make a Monster 100% happy. On certain Islands, some Monsters have more than four likes available, but it is not possible to make a Monster more than 100% happy.
For each 25% increase of happiness, the currency production rate increases by 25% of the base value. Thus, if a Monster is at 100% happiness, it will produce currency at twice the rate of a Monster at 0% happiness. If the Monster is at least 25% happy, the "rate" value on the info screen will display in green numbers. Happiness does not affect a Monster's maximum income, i.e. the amount of currency the Monster will hold.
Happiness does not apply to Monsters on Celestial Island, Gold Island, The Colossingum, Tribal Island, or Composer Island. On Wublin Island, happiness is replaced by polarity, a similar production modifier exclusive to Wublins.
Requirements[]
If an item is placed within two tiles away from a Monster that makes it happier, an upwards harp-strum noise will be heard, and pink hearts will fly out of the Monster. Conversely, if an item is removed and the Monster becomes less happy, a downwards harp-strum noise will be heard, and blue broken hearts will fly out of the Monster. If a liked decoration is put away inside a Storage Structure, or if a liked Monster is stored inside a Hotel Structure, it will no longer help to make the Monster happy.
The Unity Tree nullifies the requirement to place an item near a Monster to make it happy. Items and Monsters can then be placed anywhere on the Island or within Structures and still increase happiness, allowing for greater freedom in designing Islands. However, if an item a Monster likes is sold and there are no other copies of the same item remaining on the Island, the Monster gets sad.
For purposes of making other Monsters happy, Rare Monsters behave the same as their Common versions, but not Epic Monsters. For example, Toe Jammer likes Furcorn, therefore it also likes Rare Furcorn. If a Monster is close to both the Common and Rare versions of a Monster it likes, its Happiness increases only by 25%, not by 50%.
Strategy[]
It is worth considering how often one collects currency from Monsters. For example, a Potbelly at level 15 reaches its coin limit in 6 minutes if it is 0% happy, and in 3 minutes if it is 100% happy. If coins are not collected from it more frequently than every 6 minutes, there is no benefit from increasing its happiness. Most other Monsters take much longer to reach their currency limit, so improving their happiness may increase the amount of currency that can be collected. The wiki page for each Monster shows its currency limit for each level, as well as its earning rate for each percentage of happiness. By dividing the currency limit by the earning rate, one can calculate the time needed for the Monster to reach its currency limit. The wiki page also shows the times for the Monster at level 15 to reach its currency limit for 0% Happiness and for 100% Happiness, and those times will be similar to the Monster at any level.