It's quite common for people to feel reserved around strangers and more outgoing when interacting with those they know. This behavior is often attributed to social comfort and familiarity. Many individuals tend to feel more relaxed and confident when they are around friends, family, or acquaintances they are familiar with, which can lead to more outgoing and open behavior.
Being reserved around strangers is a natural response, as it may involve uncertainty and the need to establish trust and rapport. Over time, as you get to know people better, you may become more comfortable and gradually display more outgoing traits. This is a normal part of social dynamics, and it's important to be yourself and take your time building connections with others at your own pace.