After years of searching for a soul mate, after countless dates with Mr. or Ms. Wrong, you’ve finally found your perfect match.

If only Fluffy agreed.

Pet owners entering a relationship often watch as their trusty tabby or their lovable Labrador turns psycho over the strange new person frequenting the home. Public displays of affection elicit growls, revised schedules lead to cat poop on the down comforter and, suddenly, an owner can feel as if she’s in the doghouse.

Though cats, dogs and other pets react differently to stress, a disrupted routine can shake up an animal. And few life changes outside of a new baby can alter a routine, or make a pet owner readjust a schedule, like a new lover.

That’s why a dog may display aggressive behavior — because it is generally fearful of losing more than a friend.

“The dog is looking at this new person and is afraid of losing his resources,” explains Pia McGovern, owner and president of K-9 Insight Obedience of Redwood City, Calif. (www.k9insight.com).

“This is a brutal wake-up call, but for most pet owners, the dog doesn’t love you,” she says. “He loves what you can do for him.”

The cat, more solitary by nature, in most cases is more inclined to avoid the new person than act aggressively, says Maureen Strenfel, animal behavior manager for Humane Society Silicon Valley (www.hssv.org). A feline might react to the change by stopping its use of the litter box, instead relieving itself where it feels safe and comfortable, such as on a bed or in a pile of the owner’s clothes left on the floor.

Hillary Russak, a technical writer at Stanford Linear Accelerator Center, learned that not all cats avoid the owner’s new lover when she moved into her girlfriend’s Oakland, Calif., home three years ago. Vinnie, her partner’s orange bruiser of a cat, usually scratches or bites her when she walks by.

More accustomed to the low-key behavior of her Holland lop rabbit, Toadie, Russak — and her partner — ended up seeking couple’s counseling because of the inter-species conflict. Russak and Vinnie have since entered into a period of kitty detente.

“Negotiating over your partner’s animals is probably in the top five of the major issues of your relationship,” Russak says. “It’s up there with money, sex and the other big ones.”

But no matter how animals react to the growing household, both owners and new partners need to remember that the pets ultimately are responding to a change in the owner’s behavior, Strenfel says.

“We have to make people understand that the animal is not evil,” she says. “And it is not thinking, I hate the new boyfriend, so I’m going to pee on his trouser leg when he comes through the door. They’re just animals.”