There are two primary reasons why your unsubscribe rate may be high.
1. You may not be targeting your audience correctly. For example, if you are sending a "how to" newsletter to subscribers who said they were interested in receiving coupons or other promotions, they may see your email as being of no value to them and may no longer want to receive it. Targeting based on interests is key to keeping your subscribers interested and reducing your unsubscribe rate.
2. You might be over communicating. The general rule of thumb is to send no more than one email every 10-15 days. Of course, this depends on the business you are in and the expectations you have set with your subscribers. If you are emailing more than once every 10 days, you may want to consider reducing the frequency of your emails.