"I'm pretty sure he let go of the love of his life," Harrison toldTV Guide in a recent interview.
"I'll be interested to see if he'll say he regrets it. I know she was head over heels in love with the guy. When Desiree was leaving, it reminded me of [second-season The Bachelor star] Aaron Buerge saying goodbye to [Gwen Gioia]. It's like, "What are you doing? This is wrong; you shouldn't be breaking up.' It didn't feel right."
Harrison also said Lowe's decision to eliminate Hartsock was basically all her brother Nathan's fault.
"I think it was the brother. Sean is really particular about being able to see himself as a part of the family. What you didn't see is the first conversation they had went great! Then her brother came back, did a 180 and went crazy. I heard about it later and asked if the producers did something and they said, 'No, we have no idea what the hell happened.' It made no sense," he explained.
But Hartsock may apparently have a second shot to find love on TV with The Bachelorette.
"I take my response off of Twitter and so far she has had the biggest response. [Sarah Herron] got a decent response and we'll see what happens next week, but right now Des is a big front-runner. She's the beautiful girl-next-door that you want to wish well," Harrison noted.
Hartsock's departure left AshLee Frazier, Lindsay Yenter and Catherine Giudici still in the running to win Lowe's heart. Yenter and Giudici both have a playful, bubbly side to their personalities Lowe loves that Frazier seems to lack in most viewers' minds. However, Harrison suggested it's not that Frazier's not fun but rather Lowe is looking for a girl who brings out the kid in him.
"I don't mean this to be demeaning at all, but AshLee is a grown woman and a very mature woman who's very comfortable with who she is and what she wants. She may just be a little too mature for Sean. It's not like Sean's a child, but his relationship with AshLee is a lot more serious," the host told TV Guide.
As for how serious things will get in the fantasy suites during next week's episode, Harrison doesn't promise much except The Bachelor star will touch upon his alleged born-again virginity during tonight's broadcast of The Bachelor: Sean Tells All special.
"We knew that nothing really happened in the suite, so we went there with him and he's pretty forthright about it," he added.
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