The brains intent is survival and procreation. In striving for these it defaults to and will take the most immediately, energy efficient, choice.
The brains way of increasing the odds of survival are to repeat behaviours that it has been successful with in the past. “X stimulus led to y reaction from me. I am not dead, therefore y worked successfully.”
The mind’s/brain’s way of promoting past behaviours is taking the most energy efficient pathway. (Actually it’s more of a bidding system but the result is the same and this is adequate for the purpose of this explanation).
“Neurons that fire together wire together.”
Nature is lazy, and as animals, so are we.
Generally in physics things always follow the path of least resistance. So next time someone says you’re lazy tell them it’s natural (and they will presume, for no good reason, that “natural”, equates with good or healthy or appropriate).
When we receive a stimulus electrical / chemical reactions take place in certain pathways of neurons. The more a pathway is used the less resistance is on the pathway. Think of walking back and forth through a corn field or even dense jungle – the more people who do it the less resistance and difficulty is involved as each person clears out a little more. Hence as a group they move faster and use it even more often. The faster and more frequently they’re moving the quicker the pathway becomes even clearer and the faster they can move again. This is called habit.