Inertia is a property of matter by which it continues in its existing state of rest or uniform motion in a straight line, unless that state is changed by an external force.
But inertia isn't just a force applied to matter, it also affects us: it is a lot easier to do nothing then to make decisions when the outcome is uncertain and the consequences are unknown. What if this new design doesn't work? What if we regret moving to France? What happens after I resign?
There are always plenty of reasons for not doing something - in fact, there's probably more that we haven't thought of yet, and some are even valid. Reasons not to do something come naturally, but the ease with which they come shouldn't be construed as a sign they are more valid or normal or better. It's just inertia at work.
We need to make a conscious effort to consider the reasons to do something, no matter how scary or counter intuitive they feel. We need to give them time and attention and credibility, or we risk being driven by inertia alone.