It is your right to choose to not include a license with your source code. However a lot of people have the mistaken belief that not including a license is an easy way to let anyone do whatever they want with the source code. This is incorrect.
In the absence of a license file, the default copyright laws apply. This means that you retain all rights to your source code and that nobody else may reproduce, distribute, or create derivative works from your work. This might not be what you intend.However, even in the absence of a license file, if you publish your source code in a public repository on GitHub, you have accepted the Terms of Service which do allow others some rights. Specifically, you allow others to view and fork your repository. But they still don’t have the rights to reproduce, distribute, or create derivative works.
So if you do want to share your work with others, please do consider choosing an open source license such as one of the ones featured on this site.