"Give crowns and pounds and guineas But not your heart away; (4-5)
This little gem of advice has two important points: your heart is worth a lot. Mostly only to you, though. Everyone else will waste it (except maybe your mother). So, turn out your pockets in the street. That'd be a better idea than it would be to share your heart with anyone (except your mother, of course).
Give pearls away and rubies But keep your fancy free." (5-6)
So now it's not just love on the table…it's "fancy." That's a fancy word for liking someone – or even just thinking that someone is pretty hot. Know the phrase "a passing fancy"? It's meant to suggest that fancy isn't really lasting.
"The heart out of the bosom Was never given in vain; (11-12)
Notice how the heart becomes something that you lose once you give it away? It's not something to share. A little possessive? Well, yes. Yes it is. Oh, and it's also a big downer.