Why do people do things?

Two reasons.

1. Because they have a goal, and they see this action as instrumental to accomplishing that goal. Philosophers call this “intentional action”; psychologists call it “goal-directed” behavior.
2. Because an automatic response was triggered. An example is the knee-jerk response to a tap below the kneecap. A special type of automatic behavior is habitual behavior. Habits are defined as automatic responses that are insensitive to changes in the contingency between response and reward.

Goal-directed behaviors are easy to start, but just as easy to extinguish. Habit-driven behaviors are hard to start, but also hard to extinguish.

Some behaviors are goal-directed and habit-driven — for example, brushing your teeth. These types of behaviors will be the most stable.