Unlike other books in the Bible, the book of Tehillim (Psalms) has clearly delineated chapters. Usually an obvious break is made from the previous chapter, as each psalm is introduced with a brief inscription, such as: “To the chief musician,” "A song of David,” or “A song of Assaf.”
The second chapter, however, opens with the words, “Why do the nations rage?” It lacks the usual introductory phrase. In fact, the Talmud (Berachot 10a) states that the first and second chapters are really one psalm.
Nonetheless, the content of these two psalms differs greatly. The first chapter discusses the spiritual and moral perfection of the individual:
“Happy is the one who did not walk in the counsel of the wicked... but whose delight is in God’s Torah, and in His law he meditates day and night... He shall be like a tree planted by the rivers of water, bringing forth its fruit in its season...”
The second psalm, on the other hand, speaks of the national issues of the people of Israel and their belligerent neighbors:
“Why do the nations rage in vain... against God and His anointed king? ... He Who dwells in heaven laughs. God mocks them.”
What connects these two very divergent themes? How could the Sages consider them one unified psalm?
We perfect ourselves by refining our personal traits and actions. But we cannot achieve true spiritual growth without also striving for the good of the community and the nation. And through the betterment of the people of Israel, we work towards the betterment of all nations.
On the other hand, we should avoid working solely for the good of society so that we fail to invest in our own spiritual and ethical growth.
The two levels are interdependent. True communal perfection can only occur when its members are fulfilled and perfected. And one cannot develop personally without striving with all one’s heart for the betterment of the community. Therefore, these two chapters — “Happy is the one” and “Why do the nations rage?” — are really one single psalm, reflecting on the common theme of personal and communal growth. When combined together, these two chapters begin and conclude with the word ashrei (‘happy'), as they unite these two interrelated aspects of fulfillment and spiritual growth.
(Adapted from Ein Eyah vol. I, p. 47)