Lips convey a lot of information that is often ignored or not even observed. Rich in nerves and highly vascular, the lips react in real time to the world around us. For example, every notice when people are stressed or just received bad news- their lips disappear or get smaller? In relationships, whether subconsciously or not, couples will immediately notice when their partner has issues because they can observe the tightening or compressing of the lips. Even kisses can seem somewhat different under stress (the blood flow is more restricted which affects their fullness, warmth, and pliability).
Because our lips are controlled by the limbic system, most of the behaviors that occur can be relied upon and are authentic. Joe Navarro, who to some is known as the "Nonverbal Expert" says that lip biting, is one of the ways that we pacify ourselves when we are stressed. It helps to relieve tension that may be minor or transitory. However, when something more significant is bothering us, our limbic system, in reaction to events, will compel the lips to narrow and disappear if the stress is significant enough.
We can look at other's lips to assess the level of comfort and their mood of the day. So next time you're in a meeting, with a colleague, or with your loved ones, watch those lips! They say so much and not just with words.