Irish Nationalism
Yeats was a big Irish nationalist. You see, he started writing at a time when Ireland was still a British colony (it became independent in 1922), and those Brits sure weren't treating the Irish very well back in those days. For that reason, a lot of Yeats' poetry reflects Irish nationalist themes.
We can see it in "The Song of Wandering Aengus," in fact, because it's a poem that draws on Celtic mythology. Celtic mythology, of course, is an important part of Irish culture. We can also see this Irish nationalist theme in some of his other poems, like "Sixteen Dead Men," which commemorates the sixteen Irish rebels executed by the British for their part in organizing the 1916 Easter Uprising in Ireland. We can also see it in his famous poem "September 1913," which celebrates the Irish nationalist John O'Leary.