People who unsubscribe to your email felt that the message they received wasn't relevant to them. Worse than the "unsubscribers" are the "mark as spam" users. These recipients were so unimpressed by the email that was sent they simply mark the email as spam (which gets reported to the ISP, who could restrict your message from getting to recipients at all).
It's easy to ignore the unsubscribers but, it's like that old saying: "If one person complains to the company than 100 felt the same way but didn't complain." The unsubscribers are even more dangerous than you think. Because the web is such a social tool, the fact that your message didn't appeal to them will cause each person who unsubscribes to to tell at least 10 other people about the "bad" email that was sent.
The best way to combat unsubscribers is to have an eye catching headline followed by quality information that will appeal across the range of subscribers. If you do loose a subscriber then it's up to you to bring them back before they spread the word. Suggestion: try emailing them personally and offer them an incentive to rejoin / ask them what they found not appealing. Realize that your email may go unanswered but at least you've shown that you are genuinely interested in their opinion, which goes miles towards great customer service.
If you need help with any of your email campaigns, please contact me for a complimentary email review.