I agree with what has been said here so far, but I wonder if it may go a little deeper than that:

Perhaps she feels (and I doubt it's true, but just her perception) that sex is all you want from her. If she feels that she only has one use, she's likely to use it as a bargaining tool - only initiating it when she's got something to gain from it. Also, perhaps she's not getting as much from the sex as you are....she's a little younger than you so perhaps she's still finding herself sexually speaking.

Either way, I think you need to sit her down and discuss this pattern that you've observed...she may be completely oblivious to it.

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