I'm in 2 Kings right now as I'm reading my way through the Bible. King after king did what was evil in the sight of the Lord. Their idolatry was so obvious. They put up an Ashorah pole? Of course that was a bad idea! It's so easy to look at other people around us or even at the people in the Bible and see where they went wrong. David shouldn't have been at home when his troops were at war, and he should never have gotten involved with Bathsheba. Samson should never have revealed the secret of his strength to Delilah.
I've often wondered why people make such obvious mistakes. I tend to think that if given the opportunity, I would easily make the right decision in those situations. Except when you're in the midst of things there are often lots of smaller, less obvious choices. When I was 18 I never thought it was even possible that I would make the choices I did when I was 21. But I did. I think that's one of the many reasons why it's so important to have a group of friends that holds you accountable. They can help you spot the stupid mistakes long before you're in too deep.