Casiopea: What I said is based on my own observation as I described earlier. In addition:
1. Needs: Our vocabulary needs are increasing. There is now even a discipline called Terminology
Terminology - Wikipedia, the free encyclopedia. New disciplines/needs emerge which necessitate new vocabulary – Vocabulary growth
2. Freedom and choice: Grammar rules are applied equally whereas people differ in their idiolects or choice of words depending upon their education and needs. The richness of a person's vocabulary can be a reflection of intelligence.
3. Emotions: Words are more loaded with emotions. We can express emotions with grammar but emotions are usually expressed lexically.