Seems pretty straightforward. Your heart is sweet. You make my heart feel sweet. However you look at it, the meaning gets across pretty well. I read somewhere that this term was derived from doctors in the olden days who defined a person’s character by their heart. Thus the terms coldhearted, lighthearted, heavyhearted, and the like were created. I’m not sure if this is true, especially because it’s such a simple, obvious term that I don’t think it needs a story of origin to give it proper meaning, but however it came about, it’s a pretty catchy term that’s been with us for a while.
How do I like this one? Well, I have to take a page out of Ron Weasley’s puppet voice and say: Not much. That is, I don’t quite see it as a romantic kind of sentiment. For a mother to say to her child, I find it sweet. For an Italian guy or native New Yorker to call every female in general by it, I find it totally adorable. For a kindly old stranger to call you a sweetheart for doing them a favor, it’s quite nice. But without the excuse of accents or seniority, I just can’t see it as that great a term. Maybe it’s overused now, or my experience has me biased, but it doesn’t strike me as a word used for people on equal terms.
It definitely is not something I’d want a friend calling me. (Unless, as I said, it was an Italian/native of NY.) It’s not quite as horrid as “sweetie,” but still gratingly condescending when used in certain tones. I call wounded animals and upset little siblings sweetheart when I mean to soothe them. It’s not something I’d want to hear from a lover. You are a sweetheart doesn’t sound so bad, and neither does: those two are sweethearts.
In conclusion, I’m a 50/50 with this one. Love/hate; it could go either way.