╳┋Your mind is configurable┋╳
You are influenced by the people you spend time with.
That's such an understatement that it's almost wrong.
I'll propose another way of thinking about your mind, or yourself, that doesn't draw as clear of a boundary of where you end and other people begin.
Your brain circuits are made of neurons and synapses that connect them together. If you connect your brain to another person’s brain you get entirely new circuits made from both brains. You don't even need to physically connect your brain to others. You are connected through language and the senses. It's just that in your brain the connections are direct synaptic ones and the connection to other minds is through language.
When you talk to your friend it will predictably make their neurons fire. You can control parts of their brain. They can control parts of your brain. If you think of it as one combined mind, then talking is the equivalent of thinking. Talking is just transferring neural signals over the language channel.
The more you build your shared reality the more the boundaries start to blur. The shared circuits can make you more creative, silly, free, happy, sad. It can feel like you are on drugs. And you sort of are. Like drugs other people also amplify or disrupt the normal communication between parts of your brain.
Sure, you can walk away at any point and be alone. But you are also leaving parts of yourself behind. Moments, memories, feelings, ambitions and ideas that only come to life in the combined mind and which otherwise might as well not exist.
By choosing which people to spend time with you configure your mind, your thinking, yourself.