1 Nephi 10:18

18 For he is the same yesterday, to-day, and forever; and the way is prepared for all men from the foundation of the world, if it so be that they repent and come unto him.

It makes perfect sense that God would be the same yesterday, today, and forever. The reason people and things change is due to their imperfection. For the most part, when people change, they are getting better or worse. If we were perfect, as God is, we could not improve, and we may not have the ability to get worse either. Perfection implies that you have no desire or need to do imperfectly; you have overcome that. That being the case, God, being perfect, has no need or desire to change or overcome anything. So we can count on Him being completely consistent.