A study published in 2016 found that dog owners were actually worse at interpreting the body language of dogs interacting with children than people that did not own dogs. The scientists that performed the study stated that "moreover, it was shown that having experience with a dog without any theoretical knowledge of dog behavior may be a detriment to interpreting canine language."
There are no studies like this with parrots as of when I'm writing this, but my personal experience and that of many other professional parrot behavior consultants and trainers is that we definitely see the same phenomena with parrots, and even most other groups of animals. One of my favourite quotes seems very fitting here: "practice doesn't make perfect, perfect practice makes perfect." If we practice doing something the wrong way, we will become very good at doing it the wrong way.
Spending time with parrots is very important to learn how to interpret body language, but not enough on it's own. We need to combine it with a structured framework for how to interpret what we see, or we run the very real risk of tricking ourselves in to thinking that we know things we don't. The only thing worse than not knowing something is thinking you know when you do not, especially when it directly influences the wellfare of an animal in your care.
This is why having a framework for interpreting behavior and bodylanguage is crucial both for making good observations and making reasonable (as in useful and likely to be right) conclusions based on what we observe.
2.1 PRACTICE MAKES PERFECT - OR DOES IT?
Demirbas et al., Adults’ Ability to Interpret Canine Body Language during a Dog–Child Interaction
Anthrozoös 2016, 29:4, 581-596