Rule:. The progressive is used-to emphasize the idea of continuous activity with adverbs like always, constantly, forever, continually, etc., particularly when the speaker's attitude is one of irritation or amused disparagement.
Examples of errors:
(1) Find the man who constantly turns his head around to look at girls.
(2) Therefore he always juggles with something, so he will probably throw up an apple or glass and catch it again with the other hand.
(1) and (2) occur in an essay describing a strange and amusing friend of the author's. The special tone of this use of the progressive derives from the fact that the progressive, reinforced by adverbs like always, forever, etc., exaggerates the true state of affairs. (He is not really at every moment juggling or turning his head.) This exaggeration serves to express a certain tone or attitude of the speaker towards the person or thing he is describing. This attitude may be negative and critical (e.g. He's always complaining), but may also be one of, as Leech (1971:§52) puts it, "amused disparagement. This description fits exactly the context of (1) and (2), so much so that the failure to use the progressive strikes one as quite odd.