As people start to recognize the importance of psychology, new psychological theories and hypotheses have been published rapidly. Perhaps they are trustworthy, and perhaps they aren't. As a "naïve" receiver in the sea of information, critical acceptance is required in order to avoid falling into false information: pseudoscience. Pseudoscience, sets of claims that seem scientific but aren't, may sound tempting because we tend to believe what we want to believe. For example, some people think our characteristics can be defined by blood type. According to what they described, people with blood type A are usually shy and neat while the ones with the type B are straightforward and egocentric.
I've seen a lot of books and comics with a list of different personalities of these four blood types. It makes perfect sense because they are all generalized. People with blood type O, however, can be timid when they are around strangers and outgoing when they are with friends. It doesn't mean my blood type is O if I didn't talk to a stranger I met in library today. Or maybe I was having a bad day so I didn't wanted to talk to someone I've never met. It doesn't make sense to categorize six billions of people into four characteristics, because we all have different genes and are living in different environments. Despite the fact that it isn't true, I think the reason people keep reading about blood types is because they want to fit in somewhere, and want to define themselves who they are and what their personality is in simple words. Or perhaps, people stick to these kinds of pseudoscience because of belief perseverance, which causes us to stick to our initial beliefs even when evidence contradicts them.