One of the best remedies for extreme fears (or phobias) is being forced to face one's fear until it is no longer severely upsetting. A great example of this is how one must deal with a boggart - you must make it, your worst fear, into something you find funny. Does this mean youll never be scared of it again? No, but your fear should not be as bad you should be able to control it and yourself when facing the fear.
But what does Harry fear? His boggart was a dementor, which Professor Lupin suggested meant that what Harry fears the most is fear. But does he? Lets take a look at the many things Harry has done, and what has made him most fearful.
In his first, second, third, and fourth years, he repeatedly puts his own life at risk to either keep Voldemort from returning, or to save his friends. This shows a fairly consistent pattern. Harry is not scared to lose his own life when trying to help others; hes not afraid of dying. And, although hes always working to keep Voldemort from succeeding, hes not scared of him either. Also, he constantly and willingly leads himself into scary situations (he is a Gryffindor after all; hes very brave). But is he most afraid of fear, or is it so mething else? In his fifth year, the pattern changes, showing what his greatest fear really is.
By his fifth year, Voldemort has figured out that Harry has a saving people thing and exploits it. As predicted, Harry manages to find a way to get to Sirius to save him from Voldemort himself. Once he finds out it is a trap, his first thought is that he has lead his friends to their death. He then proceeds to do everything he can to help save his friends. However, when Sirius dies, he shows his first true signs of fear real fear; fear that encompasses your thoughts, keeps you from staying alert to everything else, fear that is your sole concern fear of losing Sirius, someone he loved. He did not believe initially that he had died. Indeed, in the many previous instances when his friends have been gravely hurt, his optimism kept him from reacting with full fear; they couldnt be dead. But this time, he knows the truth. Sirius wouldnt keep him waiting; he must not be able to come back. Harry is completely encompassed by love, hatred, grief, and fear, which leads him to chase after Siriuss killer, putting himself at risk, not to help others, but out of revenge. This is a new side of Harry. And, when faced with Voldemort himself, he does something hes never done before. He just stands there, wand at his side, making no move to stop Voldemort from killing him. Although hes faced many frightening situations before, this is the first time Harry has truly faced his biggest fear losing those he loves. And when he has to deal with this fear, he shuts down; he cant face it not yet.
But this just could be a response to his grief, you say? We know Harry has a saving people thing, which could reflect his fear (much like Hermione studies much more than necessary to face her fear of failure), but think about his boggart. What does Harry hear when he faces a boggart? His parents deaths. He loves his parents, and reliving their death was the most horrible thing he had faced up until that point in his life. I believe he didnt fear the boggart itself, or fear in general, but what it made him relive. Remember, his parents deaths were (and still are) only sketchy in his mind. Although hes had his whole life to get used to them being gone, he hasnt had very long to accept that they were murdered, and to hear them being killed would be just as bad as if it was happening right in front of his eyes. I believe if he were to see a boggart now, he would see Siriuss death, or the deaths of others he loves, like Mrs. Weasley. What Harry wants the most (think Mirror of Erised), and what he fears the most, are having, and then losing, the people he loves.
People often ask, "Why did Sirius have to die?" Because Harry needed to experience the death of someone he loves. Yes, his parents died, but he never truly experienced it. Now Harry has faced his biggest fear. It did indeed paralyze him and make him act irrationally out of his strong emotions. But, hes now better prepared to face this fear in the future. Theres no doubt that Voldemort will continue to exploit Harrys weakness. Anyone who he cares deeply about is at risk, especially those outside the safety of Hogwarts. Im sure Voldemort will force Harry to choose between his life or that of a loved one; its clear which Harry would want to choose. But now that Harry knows what his life means (only he can defeat Voldemort), hes faced with the choice: Do what is right, or what is easy. This will be Harrys ultimate choice, and now that Sirius has died, he knows what he faces and his biggest fear will not paralyze him or make him act irrationally. Sirius had to die so that Harry could conquer his biggest fear.