“Despite being portrayed as a ferocious Tyrannosaurus Rex, Rex is, ironically, gentle, kind, caring, and hates any kind of argument or confrontation. Though his vocabulary is fairly strong, he is somewhat absent-minded and tends to cause accidental mayhem due to his size and obliviousness. Rex (…) fears being replaced or abandoned, either at the prospect of being thrown away or the arrival of a newer, more ferocious dinosaur toy. Despite his fears, he finds happiness and enjoyment in most things (…). With his childlike demeanor and lack of self-confidence, Rex is the most innocent of all the toys and shows the greatest desire to be loved and played with.”
Source: http://disney.wikia.com/wiki/Rex