I love the duality of Melancholy and Mirth that Milton so beautifully displays in his poems, "L'allegro" and "Il Penseroso." The two ideas are so opposing in definition, but the two poems also have similarities. Both are steeped in a very intense emotion, both talk of similar things, but see them in different light. This beautiful opposition shows Milton's ability to take two opposing elements, and while he showcases their differences, he also finds ways to blend their similarities together.
For example, Melancholy and Mirth are against each other, but don’t seem to show extensive animosity. The two poems acknowledge the other, then reject them, preferring their own way of life to follow. It’s very interesting for Milton to write two poems, displaying both sides to two very different ways of life. In a strange way, it works. They are different but they are also similar and they work together to provide a set of poems that both oppose and complement each other. He does this by making the form the same for both poems. An example of this is that in each poem, there's the acknowledgment and rejection of the other poem, the invocation of the muse, the description of that way of life, then the invocation of the muse. Both follow the same "storyline" and both talk about their way of life that brings, in their opinion, true happiness.
I believe that Milton is talking about happiness itself, by bringing together these two poems of extreme opposing forms of true happiness. Perhaps, according to Milton, both are right and both are wrong. It's that duality that I'm learning to love in Milton.