I think Confused has said it well.
Maybe to understand monogamy, you need to step back from yourself... and look at another couple. This couple loves each other, jealously guards the other from poachers, and carefully only looks at the other person.
Flirting with someone else would be a betrayal. Monogamy is a different way of looking at love. There is ONLY ONE person that you love... completely. And you will love them FOREVER. Seriously, you willl never meet someone who will love you more.
And monogamy worked okay, when people lived shorter lives.
The heart example says that he sincerely believes that he can only love one person. And worse, he believes that if you don't take care of his heart, you'll break it. That's a wonderful romantic sentiment for another mono.
(Some people are born poly. I think some people can become poly... But that is another post.)