Gravity is one of those things we take completely for granted. And there are two things about it that we take for granted: the fact that it is always there, and the fact that it never changes. If the Earth's gravity were ever to change significantly, it would have a huge effect on nearly everything because so many things are designed around the current state of gravity.
Before looking at changes in gravity however, it is helpful to first understand what gravity is. Gravity is an attractive force between any two atoms. Let's say you take two golf balls and place them on a table. There will be an incredibly slight gravitational attraction between the atoms in those two golf balls. If you use two massive pieces of lead and some amazingly precise instruments, you can actually measure an infinitesimal attraction between them. It is only when you get an gigantic number of atoms together, as in the case of the planet Earth, that the force of gravitational attraction is significant.
The reason why gravity on Earth never changes is because the mass of the Earth never changes. The only way to suddenly change the gravity on Earth would be to change the mass of the planet. A change in mass great enough to result in a change in gravity isn't going to happen anytime soon.