He breaks friendship down into meeting 10 basic needs. The more that are met, the closer the friendship. If someone else starts to meet them, the first friendship will drop off.
Dr Sedhoff's 10 needs of friendship:
1. Valued: The time you spend with a person, your investment in the friendship.
2. Noticed: Talking, speaking, being in the same room. Stonewalling is destructive.
3. Appreciated: Thanking someone for their efforts, even dinner.
4. Heard: Listening when someone wants to talk. The more we share, the more we bond.
5. Sameness: Focusing on similarities unites us.Focusing on difference makes us enemies.
6. Validated/approved: We need to hear we are doing a good job, as a parent for example.
7. Respected: Treating each other as equals.
8. Cared for: Nurturing and accepting our differences.
9. Supported: Not forcing people to be anything they are not.
10. Protected: Being there for someone.
How it goes wrong
Dr Sedhoff says he has seen many relationships go wrong when one partner becomes distant - and he believes this is often because something, or someone, came between them.
"They suspect there's someone on the side," he says. "Often there is - whether it's a colleague at work, an ex or at the moment, the internet: people are starting to engage with someone else on the other side of a keyboard who's meeting their needs."
How to fix your relationship
The Gottman Method Couples Therapy claims there is less divorce when partners share more and "move towards each other", instead of trying to guess what the other thinks.
"Some people go so far they don't feel the relationship is salvageable," says Dr Sedhoff. "If you want it to survive, great, but both of you have to be committed to meeting those needs. Otherwise, you make it very, very hard."
It may sound brutal, but Dr Sedhoff says you must "cut off other friendships" and work with your partner to rebuild the trust.
He recommends focusing on the basics of friendship: sharing intimate secrets, listening more, having weekly dates and revealing what is emotionally important to you.
What about your friend?
There's not much room for manoeuvre here. Dr Sedhoff says you may have to sit down with the mate and tell them honestly that you can't spend so much time with them.
If the friend is a work colleague, keep it professional, he adds. Don't share details about your personal lives, and definitely don't share emotional secrets.
You are allowed to keep your other friends. "Same-sex friendships are important for meeting the needs not met in a relationship," says the doctor. "Relationships are different friendships to others. If a close friend is meeting your friendship needs for the opposite sex, it prevents deep and meaningful relationships.
"Not only does it get in the way if you're in a relationship, it can get in the way of you forming relationships."
It's forthright advice. But is there a grain of truth in it?
Dr Winfried Sedhoff's book, The Fall and Rise of Women, is available now.
