Render two layers together for compositing Hello blender community,

I am trying to learn the compositor in blender but I came across a small problem. I placed the environment in the first layer and the sun in the second (sky in both disabled) like you can see in the image.
I renderd both layers together by selecting the two layers. 

The problem is that I cannot see the sun in the compositor or when I render both layers together  

I did the same thing as the video showed so can somebody help me out?

I looked around in other similar question but that didn't help me
I found the problem. 

1) I needed to uncheck "use alpha" in the compositor. 
2) add an alpha over to combine two layers.

I only don't understand why?