When I listen to "All My Loving," I hear Paul pushing back against the predominance of John's darker, more anxious songs. It's as if the ambiguous and somewhat overcast mood engendered by the preceding pair of tracks on With The Beatles - "It Won't Be Long" and "All I've Got to Do," both of which were John's - was too much for Paul and convinced him that John Lennon and all his heavy songs needed to be countered. For despite being about a couple's impending physical separation, "All My Loving" is warm and buoyant, filled with affection and promises of fidelity ("Close your eyes and I'll kiss you/Tomorrow I'll miss you/Remember I'll always be true"). It's very much a Paul song, possibly his most engaging through that early stage of his career. Had it been helmed by John, who was more prolific at the time and wrote songs fraught with tension, it would have likely had a much different feel and mood.
This shouldn't be surprising, of course: it was evident from the start of their union that John and Paul didn't share the same temperament and sensibilities. Far from it (that they underwent such divergent reactions to the premature passing of their mothers is instructive). On Please Please Me, for instance, there's a firm contrast between the likes of the title track and "There's a Place" - songs principally written by John - and some of Paul's numbers like "I Saw Her Standing There" and "P.S. I Love You." Frustrated vs. elated; introspective vs. forthcoming.
The pattern continues through With The Beatles, which features "All My Loving" as its third track. As I mentioned above, both of the two preceding entries on the album - "It Won't Be Long" and "All I've Got to Do" - came from John, and both of them have ostensibly positive outcomes and try to remain emotionally upbeat in getting there, but they each end up sounding desperate and uncertain instead. This is especially true of "All I've Got to Do," which emits hardly a negative vibe through its lyric. The music that those words are couched in, though, speaks a very different language, one that's tense and much better suited to John's dusky nature.
What Paul accomplishes, then, with the cheery "All My Loving" is a commanding, even if temporary, shift in the feel of With The Beatles. From uneasy and ambiguous to optimistic and youthfully earnest; from John to Paul.
More generally, Paul is asserting himself - his emotions, his personality, his writing style - against what was then John's relative control of the band. And to that end, he strikes a solid blow. "All My Loving" is a terrific song: sonically busy but efficient, sweet, and welcoming. It opens in bracing fashion, and most of its details, like George's country/western-style guitar solo, provide interesting touches. It's curious that it wasn't released as a single in the UK; it definitely has the goods.
Other quick notes: "All My Loving" was inspired by Jane Asher; it represents a rare instance when Paul wrote a song's lyric before the music; it hit #1 on the charts in Australia, Canada, Finland, and Sweden; and it was The Beatles' opening song for their historic appearance on The Ed Sullivan Show in February of 1964.