It cuts down other people and begrudges them what they have. It pits members of the Church against one another and divides the community into warring factions. This passage teaches us that sharing faith in Jesus is not enough to guarantee peace and harmony in the Christian community. That faith has to be expressed in ways that build up the body of the Church. Destructive behaviors have to be eliminated.
Generosity looks out first of all for the well being of the other.
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.