When's "how come" used?

Native speaker’s answer
Rebecca
"How come" is an informal phrase that is used to ask for the reason for something; how or why it happened or is true. Here, she's asking why they became the people they became. Ex: How come you left the party early? = Why did you leave the party early? Ex: A: I won't be coming this weekend. B: How come? A: I have another doctor's appointment. Ex: How come people are so kind here?