Number one fantasy

I watched this movie which is about a couple who were madly in love but when John suggested complete honesty, Amy lied and told him what she was sure he wanted to hear…that she had a lesbian experience with her best friend, until that moment he loved this dirty secret, but when he finally hears the real dirty secret, she gets the bad wrap and left her with bad taste in her mouth!  But his dirty secret is certainly not better than hers. conclusion: not all truth is good to tell. Why do people have the urge to tell their secrets?