Even if you're not watching a specific repository, you can subscribe to individual conversations that interest you.
In the upper-right corner of any page, click your profile photo.
On your profile page, click the Repositories tab, then click the name of your repository.
Click either the Issues tab or the Pull Requests tab for the repository.
In the sidebar on the right, click Subscribe.
Once you subscribe to a conversation, you receive every notification thereafter, as though you were participating in the thread.