I shall praise God among a multitude (Psalms 26:12).
While the prayer and performance of a mitzvah are always praiseworthy, it is especially meritorious when an entire community participates in it, as the Sages teach, The prayer of a multitude is never turned away (Devarim Rabbah 2).
Nothing is more pleasing to God than to see His children bound together in friendship and placing the common welfare above personal ambitions. Indeed, the Talmud states that when Jews are united, God is willing to overlook even serious transgressions.
As for ourselves, nothing is more important than realizing that no one is an island, and that we are all interdependent. The idea of complete self-sufficiency is an illusion and probably a desperate attempt at ego-building by someone who is plagued by feelings of inferiority and inadequacy.
When we do things together, we both give and receive. Others are strengthened in their resolve and actions by our participation, and we are stimulated and encouraged by theirs.
Another added benefit: Commenting on the verse, Five of you will pursue one hundred enemies and one hundred will pursue ten thousand (Leviticus 26:8), the Midrash states that when a multitude observes the Torah, their strength is not merely additive, but increases exponentially.
In working with alcoholics, I have observed the enormous power that can come from a group effort. As one recovering person said to the group, "There is nothing I could do without you, and there is nothing I cannot do when I have you."