It is called light pollution. We exert so much light into our atmosphere that the light particles bounce around and are trapped so sometimes when you go out at night you can't see the stars because you are seeing light refraction even on a clear night. That is why if you go out into the wilderness where there isn't light for hundreds of miles the stars look a lot brighter. It is really sad that many people don't know about it, and people can go for years without actually seeing things in the sky.
Also the universe is expanding, the solar system is moving and expanding, the earth is moving around the sun and on its axis, so everything is at constant change (even though it looks like it isn't). That is why some stars are brighter today or less brighter than they have been in the past.