As the demand for health insurance rises, so does the premiums. Health insurance companies have to balance their books somehow, and raising premiums is how they do it. More people will be on the rolls and more consumers will use health insurance so there are more costs involved. Because of new regulations such as non-denial due to a pre-existing condition, companies have to raise premiums to cover those new costs and new burdens.
Obamacare, with it's various mandates and it's move toward covering those with pre-existing conditions ( thosepreviously priced out of the market), will raise premiums in general. But since the law ends "individual rating" of plans and moves toward "community rating", those with pre-existing conditions could see a huge decline in premiums. Also, those with low income will qualify for subsidies which will lower their premiums within the exchanges. But overall, given the mandates, many with existing plans and who do not qualify for subsidies will see a rise in premiums (of a yet to be determined amount).