Here's some food for thought from David Cartledge's book The Apostolic Revolution:
The reason why much preaching is negative is because the preachers themselves have no confidence in God or His promises. They cannot conceive that God would bless His people, and actually have more faith in suffering and limitation than in victory and freedom. Because of this, they often oppose the idea that God delights to prosper His people in their spirit, soul, body and social areas.I asked my wise wife why she thought this was so. Her answer: It doesn't require faith at all to believe in suffering or limitation, because faith is the substance of things hoped for.