The field of neuroscience is just beginning to understand the physiological foundations of how we learn. The following is a basic breakdown of what we think we might know.
Learning is the process by which new knowledge and skills sticks to our brains. Its functional “sticky” unit is the neuron. Neurons are cells specially adapted to communicate with each other. Everything we experience is reflected in the brain by neurons communicating to form what are called neural networks. These networks can be pictured as 3-D road maps which span brain regions responsible for processing everything from the bitter-sweet taste of dark chocolate to why your neighbor is such a grump. 
As we learn, these neural “road maps” interact and shift while also fading or strengthening in relation to our experiences.
Whether it be recognizing a co-worker or changing a flat tire, learning entails the formation and strengthening of connections or synapses between neurons. Brief experiences typically leave connections tracing an ephemeral neural network. This might be envisioned as crisscrossing deer paths. Which, if left unused, fade quickly.
After repeated exposure to a learning experience, like the second time we change that flat tire, the associated neuronal connections are reinforced, resembling more a network of single lane country roads 
than deer paths. And when it comes to daily practice and expertise in a skill, one can imagine that the guy at the local tire repair shop would have the neuronal equivalent of intersecting super-highways.
This strengthening of neural network connections is the physiological basis of learning. While changing, strengthening and creating new brain pathways tends to get more difficult with age, research indicates that it is possible to maintain our ability to learn, and possibly even ward off or lessen the impact of certain types of dementia. It appears that a significant amount of age related cognitive decline is attributed to a tendency to stay within pre-established comfort zones; IE shying away from new and challenging experiences, which typically push the brain to grow (or at least not shrink as fast).
Changing habits is at the core of any desired transformation. When neuronal highways lead to places we do not want to go, we need to re-route traffic towards the goals we want to manifest. Transforming ruts into grooves is the key to living a calm, clear and connected life.