It encourages people to come together in thought and action without consideration for its own needs. Selfishness, on the other hand, puts me first. It want what it wants, even if that means taking it away from another. Self-seeking is divisive and conflictual by definition. Taking second place and letting others have the advantage is totally foreign to the selfish heart.
Generous people are the answer to a pastor's prayer.
They help the community work as one. They bring a sense of cooperation to the local community. They are artisans of peace and tranquility, real agents of the love of Christ for His Church. Selfish people, on the other hand, bring division, suspicion, strife. Enough people like that can lead a community into a state of ongoing turmoil, of unrelenting conflict, whether the community be a parish, a diocese, or even a whole country.