Usually it's a lack of fit. Either the content doesn't fit, the timing doesn't fit, or the expectations don't fit your recipient.

Think back to the reasons you've unsubscribed and I'm sure one of the above applied to every single one. Unless you unsubscribed by accident.

You can head off lack of fit by being ultra-clear from the beginning about what a subscriber can expect to receive from you. Don't have a subscribe widget as vague as "Receive daily updates!" Try to give them a sense of what you're offering in a few words.

Then, in the first email to a new subscriber, introduce yourself and your company and set expectations.

Tell them about the content you have planned for them.

Tell them the planned frequency of your emails.

Tell them if you plan to sell them something, whether directly or indirectly (if that's not the reason the person subscribed in the first place).

Make sure that first email reflects your brand in tone and format.

During the subscriber's stay, be sure your email content remains true to what you described in that first email. If you need to change, tell your subscribers in advance.

This way, when people unsubscribe, you'll know it's not you. It's them. Perhaps something changed in their lives that made you obsolete or irrelevant. So they ran into a lack of fit, but not because you're unfit.