What motivates Lelia to take these private trips knowing that her marriage is in trouble?

In the novel, Lelia is Henry Park's wife. At the beginning of the story, Henry tells us that all is not well in his relationship with Lelia.

During the weekends, Lelia has taken to going on trips by herself. The destinations are varied. One weekend, she takes a trip to see her friend, who purportedly makes cheeses for her local city's street markets. Another weekend sees her flying west to El Paso (where they first met as a couple) or Montreal, where she mingles with the French-speaking locals. Soon, Lelia begins to take day trips to different parts of New York City every day.

It is only after Lelia leaves to travel between the Italian islands for the winter, that Henry begins to realize the import of her actions. After the death of their son, Mitt, Henry had grieved differently from Lelia, so much so that Lelia found it difficult to relate to Henry's apparent coldness and passive indifference in the face of tragedy. Before she leaves him, Lelia makes a list of Henry's attributes. Among them is the assessment that Henry is "surreptitious," a "stranger," a "traitor," and a "B+ student of life." Henry's tendency to suppress his emotions has made him a stranger to his wife. To Lelia, Henry has betrayed his humanity by refusing to acknowledge that some emotions must be expressed in a healthy relationship.

To Lelia, a woman who "can't hide a single thing...she looks hurt when she is hurt, seems happy when happy," Henry's reaction to Mitt's death is devastating; he is as "serene as Siberia" in the face of heartache and unimaginable grief. For his part, Henry has always been a man who controls his emotions perfectly; his line of work demands it. He laments that "on paper, by any known standard," he was "an impeccable mate." He knows that he "cooked well enough, cleaned enough, was romantic and sensitive and silly enough...made love enough...bull-headed and dull and macho enough," but he also knows that he failed his wife miserably when it came to grieving with her.

So, Lelia's extensive travels is perhaps a way for her to alert Henry to the serious problems in their marriage. In her estimation, the list is a necessary element in addressing the conflicts they have both been grappling with since their son's death. Also, we can't rule out the fact that, from Lelia's viewpoint, the possibility of Henry changing is slim. In that light, her trips may well have been representative of her discouragement and disillusionment at the impasse between them.