JOEL - Here are the two scriptures in question:

Moroni 7:45
"And charity suffereth long, and is kind, and envieth not, and is not puffed up, seeketh not her own, is not easily provoked, thinketh no evil, and rejoiceth not in iniquity but rejoiceth in the truth, beareth all things, believeth all things, hopeth all things, endureth all things."

1 Cor 13. 4-7
"Charity suffereth long, [and] is kind; charity envieth not; charity vaunteth not itself, is not puffed up, Doth not behave itself unseemly, seeketh not her own, is not easily provoked, thinketh no evil;
Rejoiceth not in iniquity, but rejoiceth in the truth;
Beareth all things, believeth all things, hopeth all things, endureth all things."

Of course we can no longer ask Joseph Smith, or Mormon, or Paul why these two passages are similar, so here is a list of possible explainations:

1. Joseph Smith plagerized the Bible as he was writing the Book of Mormon. Of course we can dismiss this as a possibility, assuming we have enough faith that Joseph Smith actually translated it from gold plates through the power of God as he claimed.

2. Bible scholars have labeled this particular passage "the Hymn to Charity", and, according to Hugh Nibley, it was shown early in this century by a number of investigators working independently (A. Harnack, J. Weiss, R. Reizenstein) to have originated not with Paul at all, but to go back to some older but unknown source that both Paul and Mormon had access to(Paul - perhaps from Old Testament manuscripts; Mormon - through the brass plates brought by Lehi). Both may have been quoting from the same ancient source. (See Hugh Nibley (1989), The Prophetic Book of Mormon, Salt Lake City: Deseret Book and FARMS).

3. It is also possible that the Savior could have given a sermon on charity on both the eastern and western continents, which means both Paul and Mormon may have simply been quoting something that was said by the Savior in both places.

4. Others have speculated that Joseph Smith recognized Mormon's teachings as similar to Paul's and likely used the biblical text where it agreed in meaning with the plates. Where the meanings differed, he followed the plates.

Since the ultimate source of the teachings of both Bible and the Book of Mormon is Christ, one should expect parallels between Book of Mormon and Bible passages.