Wow... Today, I stopped to watch the video and pay full attention to it... ! The lyrics are about the Aristophanes' speech - Plato... Really cool!
In your first example, "the origin" is definite. In your second example the definite article points to "love" instead.
While "kjærligheten" is definite in Norwegian, the natural way to translate it to English would be without a definite article.
However, this could also be used to refer to a specific love, right? Example: Jeg respekterer kjærligheten til Lars og Ann. Jeg vil vite kjærlighetens opprinnelse!
In theory, yes, though it's not a very likely phrasing for that - and would need quite a bit of context to work. The English translation would be "their love" in that case, but I don't think allowing it here (without context) is a good idea.
Okay, fair enough. I will file this with words like 'naturen' and 'livet'.
In the first sentence, "the origin" is definitive. In your sentence, "the love" is definitive. Each sentence refers to two different things.