Conflation of Specific and General
We look at a specific instance of something we don’t like, and we expand that dislike to the more general idea of the thing. This is a form an anecdotalism, where we think the single instance of a thing is an accurate and global reflection of the larger thing, and all instances of the larger thing are the same as the one instance we have experience with. We fail to separate the two and fail to understand that the larger thing might be interpreted into smaller things many different ways, some of which we might like more than others.
Examples
When people say “CrossFit is bad,” they might really be saying, “I had a bad experience at this specific CrossFit gym, so I am now convinced that all of CrossFit is stupid.”