The generally accepted history about the origins of this song is that Paul McCartney wrote it for Julian Lennon, to help him through his parents’ divorce following John Lennon’s affair with Yoko Ono. There have been a couple of other theories about it, though. John Lennon believed that Paul wrote it for him (of course he did, because he was a megalomaniac) as a subconscious encouragement on Paul’s part for John to pursue his relationship with Yoko. The other theory is that Paul wrote it to himself, and he has acknowledged this to be partially true. This is the interpretation I like the best, because this song has always sounded to me like an inner monologue (rather than advice to an outside person), the theme of which being the ideas of self-sufficiency and believing in oneself, while at the same time having the courage to take the chance to rely on others.
Music critic Tim Riley wrote of the song: “If [it] is about self-worth and self-consolation in the face of hardship, the vocal performance itself conveys much of the journey. He begins by singing to comfort someone else, finds himself weighing his own feelings in the process, and finally, in the repeated refrains that nurture his own approbation, he comes to believe in himself.”