Johnny Rotten

I think this is what it comes down to. It's who you share the most of yourself with, not just physically and emotionally, as you do with an SO, but also your hopes and fears and dreams. Whether or not it's necessary for you to have your SO be your best friend depends on what kind of connection you need to have. And often you don't need to explicitly communicate your thoughts in order for your SO to understand you and love you.

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"The idea that money doesn't buy you happiness is a lie put about by the rich, to stop the poor from killing them." -- Michael Caine