It is not uncommon to see your dog licking their lips. The most common causes are that they have just eaten, they are hungry, or that they are getting something out of their mouths such as dirt or sand. However, there are some reasons that you need to be aware of that are not so common. Therefore, the first thing that you need to do is to see when and how often your dog licks their lips, and that will give you a better idea of why. Unfortunately, it is not always because they want something that they have smelled.
If their lip-licking is new and very frequent, you may want to get them checked at the vets. They may have oral problems such as ulcers or teeth issues, they may feel nauseous, or other more severe problems such as acid reflux, kidney diseases, or obstructions in their intestines. However, if it is more common when you meet them for the first time in a while, such as when you come home from work, it is more of a submissive and relaxed meeting gesture. So, take note, then take action, if it is needed.