Why is desire the root of suffering?

I fail to understand how this teaching would ever help man become productive. Please help me understand this. Not everyone seeks nirvana and some people want to better the world, not only for themselves but for their family and countrymen. They do so either out of desire to reduce others' suffering or by seeking alleviation for their own sufferings. I've read a lot of self help books which emphasize the need for ***desire*** as such, and faith in oneself to achieve that desire. Is that not how every living being functions - driven by desire? Every business that is built is motivated by a desire. Every material possession is obtained through desire. We can extrapolate that to the spiritual realm by saying that man travels, meditates and watches his thoughts driven by a desire to attain spiritual peace and fullfilment. Thus I ask, how can desire be the enemy? Shouldn't it be greed instead?