President Obama is struggling to quit smoking for good, joining the millions of Americans who know what an uphill battle it is to give cigarette butts the kiss-off.

"This is not something that he's proud of," said White House Press Secretary Robert Gibbs at a Dec. 9 briefing. "He knows that it's not good for him. He doesn't like children to know about it, obviously, including his. And I think he has worked extremely hard."

So why is smoking so difficult to give up?

Inhaling a puff of smoke from a cigarette sends nicotine molecules zooming up into the brain within seconds. There, the nicotine grabs hold of receptors on brain cells and releases a wave of dopamine, the brain's feel-good chemical, bringing feelings of pleasure and comfort.

Besides dopamine, smoking also activates the release of other molecules in the brain that make you feel good, including chemicals called endogenous opioids, which heighten positive feelings and subdue negative ones, according to 2004 research from the University of Michigan.

And cigarette companies haven't been making it any easier for people to kick the habit. USA Today reports that over the past decade, companies have made changes to the design and ingredients in cigarettes to make them more alluring to first-time smokers, and more addictive for long-time smokers.

These changes include adding ammonia to the tobacco, which converts nicotine into a form that gets to the brain faster, as well as adding holes to cigarette filters that allow people to inhale smoke more deeply into their lungs, the paper said. Other additions, such as sugar and "moisture enhancers," reduce the dry, burning sensation of smoking, making it a more pleasant experience — especially for new cigarette smokers.

When you stop smoking, and your nicotine receptors stop being activated. Essentially, you're not getting as much dopamine as you're used to, which can cause feelings of withdrawal. In fact, a 2002 study in the Journal of Consulting and Clinical Psychology showed that people who had quit cold turkey had lingering feelings of depression and tension 31 days later.

Adolescents' bodies are even more sensitive to nicotine, and therefore, they become addicted more easily than adults, according to the U.S. Department of Health and Human Services (DHHS). This helps explain why, every day, approximately 4,000 teens try their first cigarette and 1,000 teenagers become daily smokers.

For those hoping that switching from regular cigarettes to filtered, low-tar, or "light" variations is a healthier choice, research has shown these alternatives do not reduce the overall risk of disease among smokers, and may actually hinder their effort to quit, according to the DHHS.

This article was provided by Life's Little Mysteries, a sister site to LiveScience.