Rivers form their course by means of a process of repeated erosion. As rain falls on a hillside, that rain gathers into a rivulet. That rivulet gradually creates a channel for itself, and gradually grows into a stream. Eventually, as the channel of the stream is deepened and widened by repeated flows of water, the stream becomes a river which develops well-defined banks and a definite course.
In the same way, our actions become habitual. These habits become part of our personality and we take these habits with us from life to life in the form of mental formation or habit energy.
This is otherwise illustrated as “Watch your thoughts; they become words. Your words become actions. Your actions become habit. Your habits become your character. And your character becomes your destiny.”
Our actions in this life are conditioned by the habits which we have formulated in our past during this life, and possibly over the course of previous lives.
This is what practice is about.