Forrest Gump's mother was wrong: life is not like a box of chocolates. It's easy to choose a box of chocolates that are all of one type, while life puts a range of flavors on your plate whether you want them or not. Alas, the one flavor guaranteed to show up in everyone's diet is pain, but most people also find some happiness. It is interesting to see how this dichotomy is reflected in songwriting.
Music differs from movies in that movies tell a story that almost always requires a central conflict around which everything forms and moves. Conflict is hardly associated with happiness, so a representation of pain or difficulty is sure to inhabit the overwhelming majority of movies, happy ending or not. But songs don't have to tell a story (though they certainly can). A song can simply be a musical snapshot of a feeling, time, or place. That makes it possible for a song to be exclusively happy or sad or even a sophisticated blend of the two.
The blues, for instance, gain a lot of their expressive power from a frame of reference that is grounded in the difficult realities of life. Yes, there are happy blues, but the very idea of blues comes from sadness and hard times, and their appeal is their worldly-wise perspectives that lie under their lyrical surfaces.
We find a full spectrum of moods in pop and rock lyrics, but they seem disproportionately skewed toward unhappiness (I also find that to be true of movies and novels). One reason for this is the aforementioned commonality of pain. As the saying goes, “Misery loves company,” and because everyone has problems, songs that express pain resonate more broadly than happy songs.
I also think it is easier to wax poetic about trouble and garment-rending travail than to write a happy song without sounding sappy. I've come to see it as a worthy challenge to write songs with positive feelings or messages that don't sound syrupy or preachy. It isn't easy.
One way to write songs that have a positive message is to entirely ignore the complexities of real life and write very basic lyrics. For example, Motown in its heyday excelled at producing these kinds of songs. The risk occurs when an author is striving for the simple while trying to avoid the simplistic, which can just sound dumb.
Another approach to writing happy songs that are convincing is to acknowledge the difficult reality while focusing on an overall positive viewpoint. Robert Hunter's lyrics for the Grateful Dead's “Ripple” and “Box of Rain” — both from the album American Beauty (Warner Bros., 1970), recorded while the band was going through serious life changes — carry the message that life is tough but beautiful nonetheless. The challenge with this type of song is to maintain a balance: too little “bummer factor,” and it sounds fluffy; too much, and the song doesn't express a positive outlook.
Another approach that sometimes works is to take an impressionistic tack, in which the lyrics might not be literally coherent but are composed of a series of images that, individually and collectively, convey a feeling that is upbeat. John Lennon used this particular technique to good effect in “Across the Universe.”
Then there is inspired silliness. It's difficult to hear Bob Dylan's “Rainy Day Women #12 and 35” or Louis Armstrong's apparently well-lubricated “Lonesome Road” without at least a chuckle. The interesting thing about the Armstrong cut is that the lyrics themselves are sad, but Armstrong casts them into such a wacky performance that the overall effect is giggly.
There is no more a pat answer to the question of how to write a good happy song than there is to the question of how to be happy. But I put it to you that it is harder than writing sad, mad, or gloomy songs. That doesn't make happy songs better than sad ones, but it does make writing them an interesting artistic hill to climb.