Being devout, she took that decision to the Lord in prayer, and after much seeking, felt that the answer was no, she should not. She was not particularly happy with that answer. Friends of hers were departing for missions, it looked like good, praiseworthy, and exciting work to be engaged in. She would have loved to have received a "yes".
It was also difficult for her because in her circles of friends, serving a mission was considered one of the most devoted and praiseworthy and positive decisions one could take. Women who did so were admired and respected. Why, if it were an option, and she was willing, would the Lord tell her no?
We live in a world where it is considered best to do as many good things as possible; that passing up opportunities to do a good thing is something that you will regret; or if you cannot do all the good options at least do the options that are perceived as the most noble and good, if you have to pick and choose.
But Jesus didn't say do as many good things as you can. And he didn't say that the ones you think are be most noble and good are the ones you should do. He said, do good, and the standard he held for that for himself in that regard was "doing the will of the Father".
Jesus could have done many good things in his lifetime that he did not do. He focused on the one thing that the Father had indicated was most important for him to do, which, in his case, was making it possible for us to be "raised up at the last day".
John, chapter 6
39 And this is the Father's will which hath sent me, that of all which he hath given me I should lose nothing, but should raise it up again at the last day.
40 And this is the will of him that sent me, that every one which seeth the Son, and believeth on him, may have everlasting life: and I will raise him up at the last day.
That's a much bigger calling than any of us have, but the principle applies to all of his disciples. Our best task or purpose at any given stage of our lives, in God's eyes, is the one he sets for us. And that is not necessarily, every time, the one we assume would be the most noble or admirable.
The goal is not to do the tasks we think are the most noble and good. Nor is it to take advantage of every opportunity to do good. Rather it is to tackle the tasks He calls us to do. And to celebrate when our sisters and brothers do theirs.