It has to be glitch/bug, I once placed 2 sims by the table with free will at full and well... after I made them talk they just talked and talked, mind to say energy, bladder, hunger and hygiene were close to max that time on both of them. They stopped only when one of them really had to take a bathroom break and of course they didn't move on to the next topic. Like samoht04 said, it's actually quite good and easy way to make friends/improve relations but... you know.
[Edited for vocabulary and spelling. - Swede]