What makes some stars brighter than others?
There are two kinds of brightness. If you mean why do some stars look brighter to us, a lot of the reason is that many of the brightest stars we see are simply closer to us than other stars. So they seem brighter.
But there are also stars that are quite far away---and they can also seem bright. Usually, the bigger the star, the brighter it is. So a large star will be brighter than a small star at the same distance.
When you look up into the sky, you are seeing stars of all sizes and at all distances. And that's why they have such differences in brightness.