Behavior is just how any given person reacts to any given stimulus. It is almost impossible to find two people who will react the same way to everything and there are so many factors involved that predictions about their behaviors are little more than speculations. A star is really quite a simple thing compared to the human brain. A star has no consciousness, no beliefs, and no capacity for independent thought or action. Stars always follow the same unchanging rules, thus, their behavior can be predicted by applying those rules.
Human behavior is a combination of learned responses and conscious decision-making.