Next to bounced and unsubscribed readers, there can be a reduction in your active subscribers. Sometimes your subscribers move from active to inactive. It might go rather unnoticed, but it is a tangible reduction in your list size.
The main reasons for this phenomenon? Subscribers may have changed their email address without notifying you. Or maybe you’re sending them too many emails that are not relevant to them. This can easily be avoided: focus on quality of emails instead of quantity.
As long as you keep your emails highly relevant, your reader will remain active and interested. Think about adding a frequency option for your subscribers and give them easy access to profile update forms.