Every new year, the resolutions fly. This is the year to get in shape, exercise daily, run a marathon. And then the motivation fades.

Why does it seem so consistently, and annoyingly, harder for some of us?

“Neurochemicals in your brain,” said Vikram Chib, assistant professor of biomedical engineering in the Johns Hopkins University School of Medicine. “They influence your willingness to exert.”

His new research suggests some people naturally have more of these chemicals, specifically dopamine, which is central to a person’s mood and sense of motivation and reward. Those with higher levels may remain more likely to stick with a fitness routine because the exertion feels like less of a burden.

Chib says understanding this may help people set more reasonable fitness goals for themselves.

For the research, Chib looked at MRI studies to map the area of the brain responsible for the feeling of exertion in people with Parkinson’s disease, which causes loss of dopamine-producing cells in the brain. His findings, published online April 1 in NPG Parkinson’s Disease, showed those with low levels of dopamine often overestimated their exertion during tasks such as squeezing a grip, compared to those who received dopamine supplements.