Part of it has to do with wanting, and part of it has to do with needing. For "needing" you must consider the context under which the second amendment was written. It was not written so dudes could shoot deer, and it wasn't really even written for the sake of self defense. It was primarily written so that the citizens of a nation can uprise with necessary force if a totalitarian government were to come into power and take advantage of the people. How can you fight a government with handguns and older rifles when your government has fully-auto modern rifles? Not a fair fight, and not a threat. This is why individuals need the right to carry modern rifles (a more accurate term than "assault rifles" for these weapons).
Secondly, you should talk about the "want" aspect. People don't "need" dogs, they want them. These are animals that we have domesticated and give us happiness, exercise, protection, but also sometimes do bad things like bite people. None of these things are necessary for our daily survival. Some dogs look scarier than others are no more dangerous than others unless they are not trained well.
Similarly, weapons termed "assault rifles" LOOK scarier than others, but statistically speaking they aren't any more dangerous than bolt action rifles or handguns. They are in fact responsible for an average of 3% of firearms-related crime nationwide, and have been for years now. Should we outlaw all scary looking dogs even though they aren't responsible for more crimes? That sounds pretty stupid.
Anti-gunners greatest fault is their inability to examine facts and use relationships to gain a better understanding on a situation. Maybe they're born with defective left brains?