Something or someone that never changes, that stays the course, and that is more like a rock than a leaf blowing in the wind shows constancy.
Have you ever noticed how some people change their minds every day? Someone like that isn't showing constancy. In parts of the world where the weather changes frequently, the weather has no constancy, whereas in San Francisco, where it's about 70 and mild for ten months of the year, the weather has constancy. If you take constancy — a good thing — too far, you have stubbornness — an undesirable quality.