There used to be an old saying about assumptions: "Never assume; it makes an "ass" of "you" and "me."" This piece of wisdom seems to have fallen by the wayside.
Assuming can lead to social bloopers. But it has worse effects: It can lead one into dangerous situations. One of the most common assumptions, in my observation, is the one held by many women that if a man seems like a nice person, he is a nice person.
Women are socially trained to be "nice" and accommodating and to not be "critical." Although many women don't adhere to these norms and stereotypes, all women are to some degree affected by this social conditioning.
Even with my decades of professional experience and training, I don't always know in the first session that a patient I'm working with is a sociopath or a dangerous person. Sometimes such individuals reveal themselves quickly, but others mask their dangerous traits--in a psychotherapist's office, the disguise is in the form of other symptoms. In real life, the disguise is in the form of charm, sociability or verbal skills.
Both men and women can be sociopaths. In women, being pretty can be the "disguise" (disguises aren't necessarily deliberate). There's been research showing that physically attractive people are assumed by many people to be morally superior. This assumption--like most assumptions-- isn't based on facts.
Making assumptions won't make you popular with other people. It might sound like a compliment to say to someone, "But I thought you were much younger!" but depending on the person, it might not be heard as a compliment. It might also make you look like you're not very observant or that you are being dishonest.
People respond more positively to questions and interest in the responses. Most people, particularly women, are attracted to others who show interest in finding out who they really are. The generic compliments some men give women on dates might puff up the self esteem of a woman who is insecure, but a woman with better self esteem responds more positively to questions about who she is and what she likes.
Professionals, including mental health professionals, aren't immune to assumptions. An amusing (or not) example is the following experience I had while working in a residential treatment program for men with mental health and substance abuse problems, in the 1990s: One of my tasks was to write up summaries for patients' post-discharge housing resources. I scanned the initial assessments of one such patient, and saw that the psychiatrist and other professionals had consistently described the patient as "white" or "Caucasian" in their assessments. I didn't know the patient well as I didn't do individual counseling at that job, but I had been trained to never make assumptions and to never go by another clinician's assessment. "How would you describe your ethnic or racial background?" I asked him. He looked confused. "I'm black," he responded. I decided not to tell him that he had been assumed to be white by numerous persons.
I don't assume I know people's race, their age, or even their gender, unless the facts are extremely obvious. I certainly don't assume I know their personalites just by looking at them. It's always better to act from the standpoint that you don't know than to assume you know things about people based on outward appearances.