The definitive Greek lexicon describes makrothumia as a 'state of remaining tranquil while awaiting an outcome' or 'state of being able to bear up under provocation' (BDAG 612). The word is comprised of a prefix meaning 'far, from afar' attached to a root meaning 'wrath.'
After an extended beautiful exposition of why we should be long-suffering as believers and what it looks like, Edwards lists four motivations to makrothumia. Indented paragraphs quote Edwards.
1. The example of long-suffering in Christ.
He was a meek spirit and of a meek, long-suffering behavior. . . . He meekly bore innumerable and very great injuries from men. (197)2. The unavoidable need to be long-suffering.
If we are not disposed meekly to bear injuries, we are not fitted to live in such a world as this, for we can expect no other than to meet with many injuries in this world. We do not live in heaven. . . . We live in a fallen, corrupt, miserable, wicked world. . . . The world has even been full of unreasonable men, men who will not be governed by rules of justice, but are carried on in that way in which their headstrong lusts drive them. . . . And therefore those who have not a spirit of meekness and calmness, and composedness of spirit to bear injuries in such a world are miserable indeed. (198)3. The untouchability of someone who is long-suffering.
He who has such a disposition and frame of mind established that the injuries he receives from men do not exasperate his spirit, or disturb the calm of his mind, lives as it were above injuries, and out of their reach. He conquers them and rides over them. (199)4. The glory of being long-suffering.
This spirit of Christian long-suffering and meekly to bear injuries is a true greatness of soul. It shows a fine and noble valor for persons thus to maintain the calm of their minds; it shows an excellent inward fortitude and strength. . . . It is from a littleness of mind that the soul is easily disturbed. . . . He that possesses his mind after such a manner that when others reproach him and injure him . . . can notwithstanding maintain in calmness a hearty good will to his injurer . . . he herein as it were manifests a godlike greatness of soul. (200-201)--Jonathan Edwards, 'Long-Suffering and Kindness,' in The Works of Jonathan Edwards, Yale ed., 8:197-204