grateful (adj.)

1550s, "pleasing to the mind," also "full of gratitude, disposed to repay favors bestowed," from obsolete adjective grate "agreeable, pleasant," from Latin gratus "pleasing" (from suffixed form of PIE root *gwere- (2) "to favor"). "A most unusual formation" [Weekley]. A rare, irregular case of English using -ful to make an adjective from an adjective (the only other one I can find is direful "characterized by or fraught with something dreadful," 1580s). Related: Gratefully (1540s); gratefulness.

Grateful often expresses the feeling and the readiness to manifest the feeling by acts, even a long time after the rendering of the favor; thankful refers rather to the immediate acknowledgment of the favor by words. [Century Dictionary]