Why does regular repetition become a habit? Humans have the tendency to develop habits. Habits of an individual can be different, same, or unique.

Some have a habit of talking too much (chatty), drawing a box around the answer of mathematical problem, etc.

Why does regular repetition become automatic or habitual?
Here are some excerpts from Wikipedia you might find helpful for the (somewhat too-) general question:

A habit...tends to occur subconsciously.[(Butler &amp; Hope, 1995)]...[Andrews (1903)]...defined [habits as]: &quot;...acquired through previous repetition of a mental experience.&quot; Habitual behavior often goes unnoticed in persons exhibiting it, because a person does not need to engage in self-analysis when undertaking routine tasks. Habits are sometimes compulsory...Old habits are hard to break and new habits are hard to form because the behavioural patterns we repeat are imprinted in our neural pathways...but it is possible to form new habits through repetition [(Rosenthal / Psychology Today)].
As behaviors are repeated in a consistent context, there is an incremental increase in the link between the context and the action. This increases the automaticity of the behavior in that context [(Wood &amp; Neal, 2007)]...
Habit formation can be slow. Lally[, Van Jaarsveld, Potts, and Wardle] (2010) found the average time for participants to reach the asymptote of automaticity was 66 days with a range of 18–254 days...

References
Andrews, B. R. (1908). Habit. American Journal of Psychology, 14(2), 121–149. Available online, URL: http://www.jstor.org/stable/1412711.
Butler, G., &amp; Hope, T. (1995). Managing your mind: The mental fitness guide. Oxford Paperbacks.
Lally, P., Van Jaarsveld, C. H., Potts, H. W., &amp; Wardle, J. (2010). How are habits formed: Modelling habit formation in the real world. European Journal of Social Psychology, 40(6), 998–1009.
Wood, W., &amp; Neal, D. T. (2007). A new look at habits and the habit-goal interface. Psychological Review, 114(4), 843–863. Available online, URL: http://dornsife.usc.edu/assets/sites/545/docs/Wendy_Wood_Research_Articles/Habits/wood.neal.2007psychrev_a_new_look_at_habits_and_the_interface_between_habits_and_goals.pdf.