It seems at first, in Amy Tan's The Joy Luck Club, that Ying-Ying decides to marry Clifford St. Clair because she learns that her first husband has been killed. This appears to be a rational answer, but the reader must also recall that she knew she would marry Clifford before he did (with her power to know the future), just as she knew she would marry her first husband.
Ying-Ying is a woman that comes from wealth and power, but in marrying the first man who was chosen for her by her family, it might be said that she lost her power—even her identity. While that time seems particularly significant to her as a young woman, her losses may have taken place when she fell into the water (off a boat) when she was four, and almost drowned.
Separated from her family, Ying-Ying witnesses a play about the Moon Lady, and is told (as is the entire audience) that for a fee, she can have one wish...but no one hears her plea. We can assume that Ying-Ying is physically found, but somehow, something deep inside her has been lost: perhaps because of her sense that she is of no consequence—that no one is there to save her when she falls, and that no one seems to miss her during that first story. Is it any wonder, then, that she might feel she matters very little to her first and second husbands—even though Clifford is a good man?
When Ying-Ying realizes her first husband has left her, she goes to live with her extended family, under extremely undesireable circumstances. Ultimately, she tires of doing nothing, and with some money she has, she buys nice clothes, gets a chic haircut, and takes a job. When she first meets Clifford, she can tell what kind of man he is. He, however, has little knowledge of who she really is. In fact, as Ying-Ying tells it, he only learns her secrets when he becomes a ghost, ostensibly because all ghosts may know in death what they could not know in life.
Clifford wants to marry Ying-Ying, and he cares about her. He waits four years—until she is ready: and she is ready when she learns of her first husband's death. However, she is never really connected with Clifford: she goes along with him to America and quietly makes a new life as his wife. In truth, perhaps she marries Clifford because of the wish she had made to the Moon Lady that night when she was a lost child—"I wished to be found." Maybe Ying-Ying thought that Clifford might "find" her, in seeing her as no one else did, appreciating her and taking care of her.
However, by the end of the story, to hear Ying-Ying tell it, it seems that she believes she must find herself—the tiger that once lived in her, and give that tiger to her child before Ying-Ying dies, so that her daughter can be strong.