"As words have a power of affecting our hearts on all occasions, as the same thing differently expressed has different effects upon our minds, so it is reasonable that we should make this [an] advantage of language, and provide ourselves with such forms of expression as are most likely to move and enliven our souls, and fill them with sentiments suitable to them." (from A Serious Call to a Devout and Holy Life by William Law)
Even though Law here is talking about the use of our language in praying to God, to me his words define the logic behind positive thinking.
All the days of the afflicted are evil:
but he that is of a merry heart
hath a continual feast.
(Proverbs 15:15 KJV)
Working on having that continual feast,