Why did Mrs. Pan in the story "The Good Deed" want to arrange a marriage for Lili?
There are a number of reasons why Mrs. Pan wants to arrange a marriage for Lili. First of all, that is the way it is done in China. In China, it is the responsibility of the parents to arrange marriages for their daughters. Since Lili's parents are dead and she has no older brothers to take their place in this regard, Mrs. Pan offers to step in. She says, "I must be responsible for my own kind".
Mrs. Pan sees that Lili is actually grieving about not being able to find a mate. She sees Lili as being kind but unattractive, the kind of girl who would not be able to find a partner on her own. As it turns out, there is a certain amount of truth in her observation; Lili has tried, but no man has chosen her. Mrs. Pan is appalled that in America, the decision for marriage is left up to the man, and that the women are powerless and can only wait. Lili's situation affirms her belief that the Chinese way is better, and Mrs. Pan resolves to do something about it the situation.
Mrs. Pan wants to do a "good deed" that will "be counted for merit when she (goes) to heaven", but that is not her primary motivation for wanting to arrange a marriage for Lili. She genuinely likes Lili, and wants her to be happy, but by finding her a spouse, Mrs. Pan has also found a purpose in her own life. Mrs. Pan has rediscovered relevance for the cultural traditions she loves so much, and best of all, she has found a way to be "useful" once again.