I think the law of inertia (Newton's first law) which relates to matter, also relates to us. The law of inertia is defined as "the property of matter by which it retains its state of rest or its velocity along a straight line so long as it is not acted upon by an external force". (Dictionary.com)

Another way of saying this is that people don't like a lot of change, unless there's a significant enough reason to change. I think this resistance to change means maintaining the status quo for the long term as long as it's working for us. Why ruin a good thing? Even where people are in uncomfortable circumstances (like a bad job), the prospect of having to look for, apply for and adjust to a different job is often more uncomfortable than staying in the current job. And such is the case with many other examples of discomforts that we tolerate for the long term. So fear plays a part and so does an element of laziness, but these are dynamics that help us maintain the comfort of sameness.

Growth of any sort requires change, and change involves an element of risk. Those who want to improve their lives over the long term must commit to a path of ongoing change that will involve some discomfort, but which will yield a long term sense of fulfillment as we continue on the journey.