In Celtic mythology, the speaker of this poem, Aengus, is a god. But in Yeats' poem, he comes across as very mortal. For one thing, he gets old, just like we do (17). For another, he's never able to get the girl he's after (in the poem, anyway). So this poem's speaker isn't some awesome superhero with immortal powers. He's just a regular dude who falls in love with, and then loses, a beautiful girl. In this way, we can see that the poem frames Aengus as a human being, as one of us. This is one of the important ways that Yeats revises the Celtic myth.
As one of us, then, Aengus is one love-struck dude. He sees this "glimmering girl" (13) for a few seconds one day, and then spends the rest of his life looking for her—talk about obsessed. This is the other important characteristic we can glean about the speaker in the poem: love is really important to this dude. It sustains him on his long, but ultimately futile, quest for fulfillment—bad times.