President Kennedy makes intensive use of parallelism and antithesis for the sake of persuasion and to make his speech more appealing to the audience. Antithesis is a rhetorical device which is used to contrast two ideas or things in a parallel sentence. Parallel structure is a literary device which consist of a repetition of some pattern of words. These stylistic devices offer the reader a more pleasant speech as well as it makes the speech easy to understand the main idea of the thought.
Example: "Let us never negotiate out of fear, but let us never fear to negotiate."