Philosophy of mind
One reason to believe dualism is that when you ask most people what the mind is, they would say it is their personality or their soul. They would almost certainly deny that the mind simply is the brain, or vice versa, finding the idea that there is just one ontological thing at play to be too machine-like. Mental events have a subjective quality, whereas physical events do not. So, for example, one can ask what a blue sky looks like, or what nice music sounds like to a person. But it is odd, to ask what an increase in chemicals in your brain feels like. These subjective aspects of mental events are called "qualia" by philosophers.