How things become easier when we do things continuously / repeatedly?
Maybe your looking at it the wrong way. I am not sure of an exact sutta or even any sutta that says this exactly but habits passed onto another life can be because of the karma accumulated. 

For example let take the following scenario.
If your a physical trainer, you would most likely train all your life to be physically fit. Now as a trainer, you get other people (your clients) into physical training hence you do the "dana" or "giving" of physical training to lots of people. Since you have that karma of "giving out physical training", your most likely to get "physical training" as a reward in your next life. Hence there is a high chance in your next life you will be really into physical training. While you do this, you will also inspire someone else to be a physical trainer. Again giving the "inspiration to be a physical trainer". Next life, someone will inspire you to be the same as a result of your karma.

If you inspire someone, you will be inspired by someone. You make someone do something, someone else will do the same back to you. This is the karmic law. Obviously its not this simple but in a bird's eye view, roughly this is probably the cause of a habit being born on a person and then continuing until that habit is dead.

Karma is subjected to impermanence like everything else. Hence eventually it will fade away. So you won't have a forever samsara habit.