I had written about SRP in this post. In that post, I had given an example of Customer class which has multiple responsibilities. Now what is bad with a class having multiple responsibilities?
When a class has more than one responsibility, they become coupled. Coupling leads to problems such as when there is some change in one responsibility it affects the other responsibilities in unexpected ways. Now someone might ask what is wrong with that? The problem here is that, you are intentionally changing code for one responsibility and there is an unintentional effect on the code for some other responsibility. Whenever you modify code there are chances that bugs will creep in and hence your changes should be such that they only affect the responsibility you want to modify/extend. The lesser code you touch, lesser are the chances of you introducing a bug.