Erin is very young. You have to remember she was 19 went she went missing. She's was still a teenager. Many times young girls fall in love and rush to get married but then when the thrill and attention are gone, the day-to-day stuff gets boring. If her husband was gone for part of the marriage, that might make things even worse for her. I can easily see an affair happening. Young, pretty girl a long way from home in a town where she doesn't know many people. Haven't we all done really stupid things at 19???
Very true, plus, it was stated that she started dating him at age 16. Makes me think, he may have been her only boyfriend. If the affair is true, she might have gotten smitten with someone that was giving her a lot of compliments and such.