The idea of a joy being shared is doubled comes from a Swedish proverb. The entire proverb reads "shared joy is a double joy; shared sorrow is half sorrow." Given the versatility of the proverb, one could write many different essays on the subject.
Definition essay- One could write a definition essay on the proverb. First, it would be up to the writer to define the abstract concept of joy in a connotative way. While the denotative definition is essentially universal, a connotative meaning can differ for each individual.
Compare and Contrast- One could construct a comparison and contrast essay to define the proverb. The concepts of both joy and sorrow could be described and defined.
Narrative essay- One could write a narrative essay describing a time in his or her life which illustrates the concept of the proverb.
Cause and Effect- One could show how the proverb works by writing a cause and effect essay. In this essay, one could explain multiple situations where joy brought about joy, or they could use a causal chain (a sequence where one event brings about another).
Let's take a little more specific route here and determine how you can set up an expository essay about your theme: "a joy shared is made double." First, let's determine what an expository essay is. It is simply an essay that through facts (and not through opinions). Enotes provides the following definition:
a short literary composition on a particular theme or topic, usually in prose and generally thoughtful and interpretative.
The only thing that makes this a tough topic for a true expository essay is that although some joys are universal, opinions may vary about other joys in life. Still, let's explore how a five paragraph essay is possible.
The introduction might begin with the statement of the actual Swedish proverb. (See answer above.) Perhaps do some research on it as to famous people who have said it and what it means to them. Filter down to this proverb being actual truth. Then you can use the actual proverb as your thesis.
Your first body paragraph should be all about joy. You should begin with a topic sentence connecting back to your thesis proverb, and then follow that up with the definition of joy. Be sure you give examples of both universal joy and non-universal joy. Also deal with exactly how humans show joy (through smiles and body language and such). You can even explore how joys change with age and culture.
Your second body paragraph should be all about sharing. You should begin with a topic sentence, and follow it up with a definition of sharing. You should give real-life examples of sharing in regards to both different ages and different cultures. You should keep your focus on sharing between people, however, because it makes more sense with your proverb. In your proverb, it is JOY that will be shared, ... and not a "thing" per se.
Your final body paragraph should be about what it means to be doubled. Begin with a topic sentence, and follow it up with a definition of doubled. This is the paragraphs where you can connect the previous two by making the kind of real-life doubling examples as examples of SHARED JOYS ONLY. (If you prefer to make the paragraph longer, you can begin with something else, as long as the particular joy fits well).
Your conclusion should begin with the reworded proverb. Then you can go into some various examples of more shared joys made double. One example you can start with is the example of a couple having a child. That child is a shared joy and, as such, great happiness surrounds the entire family. Make sure you end your conclusion with a new idea for your readers to thing about (such as the second half of the proverb mentioned in the answer above). And, if possible, end with a good clincher.