The recaptchas are part of the anti-spam protocols built into the site. They will pop up if they detect patterns associated with spamming such as constant IP switching, rapid + frequent page loading, and other patterns/activity.
This sounds like a local issue with your browser cookies. The site does not require you to login again unless you cleared browser cookies or you're loading the site in a private browser window.
By default, the site sends an email notification regarding a new reply to a thread you're watching only if you've viewed the thread since receiving the last email notification.
However, if you prefer to receive an email notification for every new reply to a watched thread, even if you haven't...