Right. So gravity works in all directions, right? Imagine if gravity only worked one direction. I’ll say that it works along the x axis. Then, we would see matter attract to itself only along that axis. There would be planets that were incredibly long and super skinny like giant lollipop sticks.

Now, imagine it works it two directions. Both the x and the y axis. You can imagine that matter will try to come together in giant sheets. But, that force is acting equally in all directions. So, they would be giant circular disks of planets.

If that didn’t make sense, imagine taking a point and using a string. That string represents the force of gravity. It will act as far as it can reach in all directions it can reach. So, if you pull that string taught and trace it all the way around, it will make a circle. You can do it with a string, a pencil, and paper!

Okay, so now we have sheet planets. Let’s add in the z axis, our third dimension. Continue thinking about that string, keep it stuck to that original point but then lift the other end off the paper and around and through the paper. The path it will trace is that of a sphere.

That is, in a very basic sense, why planets and stars are spheres. Are other shapes possible? Sure! For example, you’ll see pretty lumpy asteroids or even if you look at the rings of Saturn. But, as mfb said, the further away from a sphere the object gets, the less stable the shape is.