Any first-year business student can tell you the key to a successful business plan is repeat customers. A Business 101 class will stress how much more expensive it is to land a new customer when compared to retaining an existing customer.

So, by their very design, the world’s best online dating site would fail. If online dating sites did what they promised 100 percent of the time -- help people find “the one” --  they would have no repeat customers and, while the word-of-mouth buzz may help draw new customers, that model is not sustainable in the long-term.

Fortunately, there is no such thing as a perfect business model, and friendly email reminders that someone has just checked out your long-dormant profile are a great way to get people to log back into a dating Web site. Whether we’re in a committed relationship or not, there’s a bit of a dopamine rush when we find out someone thinks we’re hot, and many people find they succumb to curiosity.

The problem with online dating sites -- which now account for one-in-five marriages and perhaps just as many relationship breakups -- is that they make it easier for people to give into that temptation.

Tina B. Tessina, PhD, psychotherapist and author of The Unofficial Guide to Dating Again (Wiley), says couples need to have “the talk.” There was a time when “the talk” was simply a discussion about when a couple was dating exclusively, but now it often involves promises to take down online dating profiles and close Match.com and OkCupid accounts.

“I don't think anyone -- especially women -- should assume they're dating exclusively until the talk happens,” Tessina said.  “And, I think getting notifications after you've made a commitment is way too tempting.  It's like being on a healthy diet and having someone come by randomly and wave cake under your nose.”

Take Paul, an attorney in Boston who started online dating just after his 30th birthday for all the usual reasons: he wasn’t meeting new people through work or his circle of friends from law school, he was “getting sick of the bar scene” and he was at a point in his life where he felt he wanted to meet somoene who was thinking long-term.

“It was overwhelming at first, but once I got into it, I found I was checking Match more than I was checking Facebook or my fantasy football team,” Paul said. “It’s definitely addictive.”

Still, Paul thought he would be okay when he met Jenny. They hit it off instantly and while both went on a few more first dates with other people after they met, they soon agreed they would both be better off if they took down their profiles and started focusing on an exclusive relationship.

A few months into their relationship Paul got an email saying someone had winked at him. He was curious and wanted to take a peak, so logged in and checked. Later, when Jenny borrowed his laptop and saw he had checked Match in his browser’s autofill address bar, a fight ensued.

Paul, relationship experts say, was lucky. He and Jenny were able to reconcile, but many men and women give into the temptation to do more than just look at who has been checking them out. Others will often “trade up” or at least be slow to commit for fear that someone “better” may be out there: someone who is as perfect as they say they are in their online profile.

“Who needs that?  If one of those notifications comes at the moment you're annoyed or frustrated with your relationship, it could be fatal to the relationship,” Tessina said. “There's way too much escapism, and things need to be worked out to create lasting relationships.”

Dennis Kravetz, author of the book “I Never Have Any Luck with Dating” and a psychologist who specializes in relationship issues, is also a former online dater. Even after he met his girlfriend and stopped using the site, he received weekly emails from Match.com listing his “latest matches.” With his girlfriend’s permission he checked in on his account from time to time out of professional curiosity.

“What I noticed is that some of the same women kept popping up for years.  I was surprised that these women would not have found someone by then or at least updated their photos,” Kravetz said. “Perhaps it is curiosity—’I wonder if any hotties are checking me out?’  Perhaps they are curious about what kind of people are active in Match right now.  Maybe their current partner is disposable.  They will be in a relationship with the person until they find someone better, hence that’s why they check out their matches, to see if someone better is interested in them.”

While bad for relationships, it may be good for business -- and not just the business of the online dating sites.

“The last situation is not a good one, but that is how some people think,” Kravetz said. “We psychologists have to deal with all of it.”