In general, people will unsubscribe quietly and without logging any complaints, despite all of your hard work to keep them satisfied. When this happens you need to take the opportunity to solicit feedback from them and act on it.
You must understand that talking to a disgruntled reader who has opted out probably won't help you save that email recipient, but it may help you prevent future subscribers from leaving your list.
A subscriber that you do not pay attention to, is a subscriber that you will lose; which is why you need to take all campaign feedback you get seriously, and also try to do surveys, while they still have that person’s attention. A little remedial communication is always a lot easier than getting someone back onto your list after they’ve left.