If your Handwriting has never changed from the time you learnt to write chances are you are a computer or a typewriter.
Handwriting reflects your way of thinking and thus your personality. Handwriting can reveal whether you are an introvert or an extrovert or very controlled about your emotions. It can show your energy reservoirs and your leadership. It can show your risk taking and decision making ability. It can show your time management and multi tasking skills. It can show your focus on goals and attention to details.
Now, if your Handwriting keeps changing, it means that you think and behave in different ways which makes you moody.
A person whose Handwriting keeps changing has mood fluctuations. This person is highly intelligent but moody Unpredictable and thus unreliable.
The frequency and intensity with which your Handwriting changes shows how moody you are.
There will always be slight variation in every individuals writing as every human has slight mood changes as per he situation. But frequent and drastic variation in Handwriting shows considerable moodiness.