When Stella Barrett, 4, started school, she quickly made friends with a classmate, Elan. "When I'd pick her up, the teacher would tell me she only played with Elan and got mad when he played with any of the other kids," says her mom, Lori Barrett of Brooklyn. She was amused, but also a little concerned. Shouldn't Stella have other friends, too?

Many preschoolers seem to get a little obsessed with a particular playmate, but don't start thinking Fatal Attraction. Your child is responding strongly to the first-time thrill of having a special friend, the security of playing with someone predictable, or just the joy of finding another kid who wants to play house all the time, says Patricia Ramsey, professor of psychology and education at Mount Holyoke College in South Hadley, MA. Some children simply find it easier to relate to one friend than to socialize with a bunch of kids all at once.

So if your child is happy, let her be. If the strong feelings aren't mutual, though, try setting up playdates with just one or two other kids at a time. Know, too, that preschoolers don't dwell on social rejection the way adults can: Your child's emotions are real, but also, in the scheme of things, fleeting.