Why do some girls want a guy in a relationship, but use all his emotions? And never give him anything in return?

Isn't this just using him for his time and emotional energy.
It should be a two way street, right?
It's something I was discussing with my flatmate about her relationship with someone. ( She was too drunk)
She was like I only wanted a relationship with him to have a good time.
And when I am down he can do my work.
But when she came to her end comment, she said I don't have to do the same for him.
Why do some girls want a guy in a relationship, but use all his emotions? And never give him anything in return?
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