I have a cousin probably in his late 50's now. He and his wife still act like young b-f/g-f. During family/clan gatherings, it's common to see them in a dark corner and talk like b-f/g-f...You don't know or hear what they talk about, but it's probably "puppy love" kind of talking. haha
Their children gotten use to it, along with the admiration and compliments from people about their parent's love for each other.
On the other hand, sometimes you just kind of know how much in love a couple is or not..Or if it's just lust..The tells are kind of just there..Some couples though, appear happy, but behind close doors...it could be chaotic..haha
But back to the subject, yeah- sometimes it's the overly friendly, kind, caring person to watch out for. However, there are very rare instances where some people really are that genuinely friendly, kind and caring- too good to be true, because they are so rare...this is where you really gotta develop a keen sense of discerning Real Gold from Fool's gold. haha I heard that if you burn gold, it only shines brighter, where fake gold, turns dull or darken...ahaha