A thought is merely an idea. The label “good thought” or “bad thought” comes to be when we interpret the idea: we either like the thought or we do not like the thought.
Bad thoughts could indicate parts of one’s experience that are in need of resolution/healing. To illustrate, let’s say you recall a time when you totally embarrassed yourself at work. That incident happened; it is over. Yet, if the memory resurfaces and you feel a charge (i.e., a strong feeling) in response to that memory, it might indicate there is some part of that incident that has not been resolved.
The way our mind “thinks” is a product of our thinking process. Said differently, our thoughts reflect what we think about. The more attention we pay to certain thoughts, the more likely they are to grow. We also subtly pick up messages through images we see, words we read, and in conversations.
If someone reminisces about a bad relationship in the past, for instance, they are giving attention to those thoughts. Moreover, if you interpret those thoughts a certain way (e.g., with a charge of anger, sadness, etc.) you are growing these types of thoughts in your head.