MSNBC: When Jana Dolnik and her husband, Ladia, signed up for horseback-riding lessons, they never expected that their new hobby would come with an added bonus: sparks to rekindle their decades-long marriage. As Jana watched Ladia tenaciously grappling with his new challenge, she realized that even after 29 years of marriage, her husband still had some surprises. And suddenly she could recall the spirited and intelligent man she'd chosen to marry so long ago.

"He looked so happy," says the 57-year-old doctor from Palo Alto, Calif. "I was astounded. And, oh my God, you should see him on a horse -- he looks so good."

Experts say that shared challenges and exciting diversions are what make relationships hot long after the wedding gown has been packed up and stored away. And the opposite, boredom and a dull, daily routine, can kill a marriage, squashing intimacy and romance.

In fact, couples who say they are bored tend to grow increasingly unhappy, according to a study published this month in Psychological Science.

Most research on long-term relationships has focused on eliminating problems such as conflict and tension, explains the new study's lead author, Irene Tsapelas, a researcher in the psychology department at the State University of New York at Stony Brook. But surveys have suggested that boredom may be even more corrosive to a relationship, she adds.

For the new study, Tsapelas and her colleagues followed 123 couples who applied for a marriage license from Wayne County, Mich. It was the first marriage for all of them. The couples were interviewed about their relationships after seven years of marriage and again after 16 years. They were asked about their marital satisfaction and a series of relationship questions, such as: "During the past month, how often did you feel that your marriage was in a rut (or getting into a rut), that you do the same thing all the time and rarely get to do exciting things together as a couple?"

The researchers found that boredom at the seven-year mark strongly predicted future unhappiness and loss of intimacy nine years later.

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