Why do people get into relationships when you can be just as happy alone?

I see so many people ask questions about issues in their relationships and how hard it is, but single people asking how to get into relationships are told they don't need one and they can just be happy alone.

So why are so many people in relationships which it seems like more work/volatile for the same end result of happiness?
Why do people get into relationships when you can be just as happy alone?
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