Holding hands is a universal sign of friendship and affection.
No one has yet written the definitive history of hand-holding, but the practice appears to exist worldwide among all human populations, though the meaning of holding hands varies from culture to culture. For example, in the Middle East holding hands is an expression of trust and respect, whereas in the West holding hands is typically an expression of romantic or familial intimacy.
Bob Dylan heard these lines as "I get high," and decided he liked The Beatles.
Bob Spitz, in his biography of The Beatles, fleshes out this comical little anecdote, going on to say that Bob Dylan was shocked when he found out that none of the Fab Four smoked marijuana at the time.