Because of God:

As Christians we believe that God Himself is community. What do we mean by that? Although Christians believe in One (monotheistic) God, we also believe that God is three persons. By this we mean the Trinity. (see more) In this we believe that God in of Himself has community (Genesis 1:26, Matthew 3:16-17). If God has community should not man?

Because of Man as Created Being:

In Genesis 2:18  “Then the Lord God said, “It is not good that the man should be alone; I will make him a helper fit for him.” (ESV) Often we focus on the creation of the helper and forget about the clear statement of God, “It is not good that the man should be alone;”  God says that mankind was created for relationships.

Because God has Established His Church: 

When God establishes His church He describes that assembly of people as a body. What better picture of the need for community could be shown. As Paul writes to the young church in Corinth “that there may be no division in the body, but that the members may have the same care for one another.” (1 Cor. 12:25)