Steinbeck's construction of characters in the novel is similar to Travis in that both sets of characterizations reflect individuals who really struggle in seeking belonging. The characters in Of Mice and Men really strive to belong to something, anything. George and Lennie are outsiders in their own right and their friendship enables them to belong to one another. Yet, this is challenged by the social setting in which they exist, compelling one of them to kill the other. The farmhands at the ranch exist on the ranch until they leave to find another job. This transient lifestyle helps to feed the idea that there is no real permanent sense of belonging that they experience. Travis' transience is enhanced by the taxi driver element when he "goes uptown, downtown, don't make no difference to me." Like the farmhands on the ranch, Travis does not really belong, as he is just looking for the fare and, in a larger sense, seeking something far more to which he can belong than what is in front of him. Travis' lack of emotional connection helps to forge a lack of belonging to his world, similar to the world in which Steinbeck creates. In both, the emotional transiency of its individuals is one in which individuals might seek to temporarily escape its clutches. Yet, it is one from which there is no escape.
Just to add to the above answer, both the book Of Mice and Men and the movie Taxi Driver, detail that there is no escape from the clutches of emotional hold in individuals. Since both the film and the movie mentioned involve roles of strong charcter, it is clear to see how they may relate.