Laughter stands alone, but a cry needs a why; Victory Over Japan, by Ellen Gilchrist. Boston: Little, Brown & Co. 277 pp. $15. 95.

Why, I began to wonder, near the end of Ellen Gilchrist's new collection of stories - why are some of these stories such a pleasure, and others so unsatisfying? Then I struck this passage:

''Sometimes I start telling a story that's sad and the first thing anybody says is how come? How come they went and did that way? Nobody says how come when you tell a funny story. They're too busy laughing.''

That explained it. Sad stories, when they're well told, at least hint at a rationale, a meaning to the pain. Otherwise they're just journalism. And that's what Gilchrist's sad stories seem like - well-written journalism. Her comedy is more artful.

Her style has a lot to do with it. Her sentences are utterly unadorned, and would seem banal, were they not joined together in lively, expertly modulated rhythms. It's a style well-suited to wit and surprise but ill-equipped for expansion and reflection, the ''how come?'' that sad stories require.

It isn't surprising that Gilchrist's most successful character is a child, Rhoda, the precocious heroine of the first two stories. Rhoda doesn't stop to reflect on her misfortunes - a bullying father, a bedridden boyfriend. Her heart stays light. Anything can happen to a 14-year-old. But what's caprice in a child often seems like shallowness in Gilchrist's adults. If anything can happen, then nothing makes a difference.

So it's hard to care about Nora Jane, who's pregnant, unwed, and unsure of the father; or about Miss Crystal, who's an alcoholic and a marriage wrecker. Their mistakes have no consequences and therefore seem trivial.

If fancy alone made fiction good, then Gilchrist would be a master. Her stories are full of hilarious antics. But we want to know more than what happened. We want her to suggest why.