James Allen is one of those authors I find I can read again and again and again and each time get so much out of his writings.

He says:
To maintain an unchangeable sweetness of disposition, to think only thoughts that are pure and gentle, and to be happy under all circumstances,- such blessed conditions and such beauty of character and life should be the aim of all, and particularly so of those who wish to lessen the misery of the world.

Same lesson, different author.
Control your thoughts and create you life.