I want my girlfriend to straighten her hair more often
Why do you want her hair straight?
I think it will be better for her if I can't get her hair tangled. It will be good for me too.
You shouldn't tell her how to wear her hair, it's wrong to tell your girlfriend what to do. You should support your girlfriend however she chooses to wear her hair.
I just want to help her, that's all.
i understand you're trying to be helpful, but you should just let her wear it how she wants. It's her body and it should be her choice.