Lie to a girl once and she'll start to think everything else you've ever told her was a lie. She'll start to not be able to see the difference between what is true or not. She'll start to question from now on if you're being sincere with her or you're just being bullshit. She'll start to doubt anything you say to her unless you can convince her or prove to her that it's the truth because if you can't, she simply won't believe. And don't tell her " I lied because I didn't want to hurt you. " because the truth is you lied just to save your own ass from getting in trouble.
See, when you break a girl's heart, she'll still be able to love you but when you break a girl's trust, that's gone forever.