Pretty sure we are the only animals with pink rings around our mouth. It’s flipping weird.
It probably helps with verbal language understanding as humans often use mouth movements to interpret words (see McGurk effect). But I still think it’s weird.
Big pink rings.
Okay, but I was replying to the OP. What you’re doing here is bringing in a new dimension, i.e. “plump.”
Regardless, I think I already kind of answered that when I talked about kissing. Very few animals I know of do prolonged kissing. Humans do. That, and our sexual attraction cycle is more or less endless, whereas with most animals, it only happens during select times of the year.
As for “why” to both things? I think it’s because humans are vastly more helpless at birth compared to most other animals. We need lots of care, and for a prolonged period. So evolution has drastically increased the mechanisms of sustained attraction between the parents via things like prolonged sensual kissing and a near-endless sexual attraction.