As a sign of how much the pop music cycle has mellowed out since the manic phase of the early 2010s, the hit songs of 2018 have brought back the ballad as a popular genre. They haven't been this slow and sentimental since the emo early 2000s, or the power ballad era of the late '80s, the soul ballads of the early '70s, or the weepy strings-section sound of the late '50s.
These 5-year periods are all instances of the vulnerable phase of the 15-year cultural excitement cycle, crashing into a refractory state after the excitement and climax of the manic phase, and before they feel comfortable enough to come out of their shell during the restless warm-up phase.
It's not just the depleted energy levels that lead audiences and performers alike toward the ballad during their refractory state. It's also the shrinking social space -- the manic phase also has soulful songs, but they are more exhibitionistic, which assumes performing before a crowd of (friendly) strangers. Once that social energy has been spent, the socially exhausted population feels more like retreating into a private space and interacting with at most the kind of people you'd meet within the home setting.
For romantic songs, that means a single person who you're devoted to and want to spend the rest of all time with -- preferably within that same isolated home setting, since widespread social stimulation is painful during this refractory state. The atmosphere is more intimate and cozy, rather than thrillingly novel as it is during the manic phase when everyone feels invincible.
Rather than list every one of the ballads over the rise-and-fall of their popularity (you can flip through the Billboard Year-End charts at the second link in this post for that), I'll just pick what stands out as the most representative of the trend during each of the vulnerable periods.
"Perfect" by Ed Sheeran (2017)
His earlier hit ballad "Thinking Out Loud" straddled the line between the manic and vulnerable phases, recorded in 2014 and soaring in popularity during '15. A few years further into the refractory state, he was better able to channel the cozy-intimate zeitgeist.
"Your Body Is a Wonderland" by John Mayer (2002)
The early 2000s is the least soulful of these vulnerable periods, but still slow, mellow, and intimate. Outside of romantic songs, there was another more soulful-sounding ballad hit -- "Drift Away" by Uncle Kracker in 2003, itself a cover whose original recording came from an earlier vulnerable phase, by Dobie Gray in 1973.
"Lady in Red" by Chris De Burgh (1987)
Most ballads from this phase were "power" -- guitar solo affairs -- which does testify to how popular the ballad is during a vulnerable phase, that even the rock gods du jour have to perform them. But it also makes them sound less ballad-y. The dream-like, ethereal, slow funk of this song fits in better with the others across pop music history.
"Let's Get It On" by Marvin Gaye (1973)
The proper contrast here is not with his hits from the previous manic phase, such as "Ain't No Mountain High Enough" from 1967, but with his big hit from the following manic phase, "Sexual Healing" from 1982. The soulful ballad from '73 is more familiar with the woman he's addressing, whereas the one from '82 sounds more like he's addressing any ol' woman who he's currently got the hots for, drawn from a broader social space, rather than the one he's devoted to. "Let's Get It On" is about intimacy, whereas "Sexual Healing" makes declarative statements of sex-positive "ideology". (Recall that sex-positive feminism peaks during the manic phase.)
And typical of manic-phase music, "Sexual Healing" has more musical phrase development, teasing with a slow intro and building gradually toward a dramatic climax, while "Let's Get It On" gets right to the mood and stays there for the whole song. Climaxing is not possible during a refractory state.
"Unchained Melody" by Les Baxter (1955)
This song was recorded and charted highly by multiple performers in 1955-'56, the version above is simply the highest-ranking one on the year-end charts. It's a good reminder for people unfamiliar with pre-'60s music of how closely the late '50s tone veered toward mawkish, weepy, pining, and dejected. For a version that has the full lyrics, and sounds closest to the version you're most familiar with, try this one by Roy Hamilton.
The most well remembered recording was from a decade later, in 1965 by the Righteous Brothers. Springing from the manic phase of the late '60s, that version is much higher in energy and comes off as more exhibitionistic than the originals. An emotional delivery that is so over-the-top does not suggest a cozy-intimate setting and an audience of one who you're already familiar with. It suggests a performance before a crowd of (friendly) strangers, where the focus is more on baring one's own soul. It's not for narcissistic aims, but to cheerlead the audience into feeling the same way, and getting a massive crowd pumped up and resonating on the same emotional wavelength. Still, it is decidedly not an intimate atmosphere, but a crowd-pleasing one.
With so many ballads filling out the 2018 chart, it feels like the genre's revival is done. We're nearly in 2019, which is similar in the cycle to 2004, 1989, 1974, and 1959, by which time the unrelenting mellow-ness was beginning to get old. You can't stay vulnerable forever. Just one more year of (decelerating) vulnerability, and then it's back into the restless warm-up phase again.
It would be interesting to see if any of these ballads from the late 2010s get covered during the next manic phase of the late 2020s, a la "Unchained Melody". I'd guess "Girls Like You," which is weepier and lower-energy than the other big ballads of 2017-'18, and offers the most room for changing it to fit a different zeitgeist in the future. Perhaps even by the same band -- just think of how different it would've sounded if recorded by the manic-phase Maroon 5 of 2013.