A friend and I were trying on jewelry in an eclectic store that just so happens to carry my books. An adorable young clerk was helping us and we all got to talking. When he learned that I was the author of It’s Either Her or Me, he excitedly told me that when he straightens up, he often flips through my book. Why? “Because,” he told me, “my mother HATES my girlfriend.”

Immediately, I went into journalist mode. How old are you? (19) Do you have siblings? (Two). Where do you fit in the birth order? (Youngest). Is your mom okay with your older siblings’ significant others? (Yes).

Okay, so benefit of the doubt here that his girlfriend, who I didn’t meet, is a sweetheart, then what do I think is happening here? Why are moms perfectly fine with some of their children’s wives/husbands, but dislike another’s?

All things being equal, a mom (remember, I’m one, too) can have a difficult time sharing her child who is:

1. The last born.

2. The first born.

3. The one she depends on (for vetting, for companionship, for peace of mind).

4. Or, the one for whom she has superhuman expectations (think Bill Gates, Barack Obama, Alex Rodriguez – hey, they had moms who dreamed, too). And in that case, no one will be good enough.

It’s confusing for the son who can’t figure out why Mom likes his sister’s boyfriend, but not his girlfriend. And that’s the case with this nice kid I met. I’ve never met his girlfriend. I’ve never met his mother. They could both be fantastic people. In fact, they probably are. But my advice to the mom would be: Pick your battles. He’s 19. If you show your dislike for this girl now, and he ends up with someone else later on who is truly terrible, you’ve lost a lot of credibility. And maybe more.

They’re kids. They’re getting ready to leave for college. Anything can happen. So be supportive. In the end, your relationship with your son will not only be more rewarding, but it will blossom.