By default, users that Like a post will not be automatically Subscribed to the post.
You can alter this default setting in Stream Configuration:
Alternatively, users can Subscribe to individual posts on a one-off basis by clicking Subscribe under the post:
Please Note: When a user authors a post, comments on a post, or is mentioned in a post, they will be automatically Subscribed to receive email notifications about that post. Users may turn off notifications about that post by clicking Unsubscribe under the post: