People constipate for a variety of reasons. Adult constipation can be caused by a number of factors. It doesn't always make sense why something is happening. Among the most common causes are: Consuming too little fibre, which can be found in fruits, vegetables, and cerealsLack of fluid intake Sitting or lying down for long periods of time and not moving enough Being inactive and not exercising often ignoring the urge to go to the toilet changing your diet or daily routine a side effect of medicine stress, anxiety or depression