Wine comes in at the mouth
And love comes in at the eye;
That's all we shall know for truth
Before we grow old and die.
I lift the glass to my mouth,
I look at you, and I sigh.
What's to say? As simple as it gets, really, Yeats creates a somewhat funny and somewhat poignant image here, of a lover looking at his or her other, taking a sip of wine, holding the object of their love in the eye, and sighing, presumably for want of love.
All I can really take from this is a sense of rueful regret at falling in love. Raise a glass, sigh while falling in love, and hope you can realize it before you're old and dead. Alternatively, I suppose it could be a sigh along the lines of "oh, you're the one I fell in love with" but I don't get that feeling. Still, despite the sigh, I find a warm sense of humor in this poem. After all, sighs can be of relief, happiness, anything. It could be someone taking comfort in catching their lover's eye.