When you follow a post, the notification bell keeps track on the activity
When you follow a post, you are notified about new comments in the notification bell.
How do I follow a post?
To follow a post, click the post's three-dot menu and select Follow post.
You automatically follow your own posts
The system automatically notifies you about new comments to your own posts - therefore, when you look at the actions from your own post, you don't see an option to 'follow' it as you don't need to take any action.