Lope de Vega in Norway

Today in Spanish class we were talking about love and obsession, the line between them, what separates them, and so on and so forth. One student tried to go down the line of reasoning that if you love someone and they don't love you back, that can be obsession. Most cases of obsession do have that element, but obviously that's not the full story. The teacher then put forth a hypothetical scenario: He has a wife, they're very happy, they love each other... then one day the wife leaves and says she's never coming back and doesn't love her anymore. If he still loves her, is that no longer love? Does love absolutely require reciprocation?

And so on and so forth. And suddenly I remembered something very strange that I saw in Norway. It would have been perfect to use in the discussion, right down to the fact that we're studying literature, even literature by the author of the line that came into my mind. I almost spoke up - but the problem is that I know the line only in Norwegian, and that might have been a bit awkward.

Ubesvart kjærlighet er som blomster dekket av rimfrost.

- Lope de Vega

I found this in Sandefjord, which is sort of the whaling capital of Norway, in a little 'castle' made of boards and such in their park. As you can see, it's laminated and stapled on somewhat randomly... You may understand that when I saw it, it threw me back a few feet. Who put it there? And why?

Strange as it was, I hadn't thought much about it in the intervening year and a half, (I can almost say years... :P), until now. At the time I hadn't even known who Lope de Vega was, but the quote came back to me word for word in class today. Now I'm very eager to find out what play it's from, and how it's said in Spanish. Google has failed me so far. Any help?