Why Tony Stark/Iron Man is a Cancer
I normally don't attribute any weight to this type of astrology, but I have plenty of friends who do. They've been kind enough to inform me over the years of what my birth date signifies about my personality traits and character. As a Cancer myself, I know that our main common component (besides being ridiculously good looking) is having a tough exterior and a delicate interior. Just like the crab that symbolizes us, the outer hard shell protects what's inside. It could be emotions, secrets, and other insecurities that we don't want bared out in public.
In the Iron Man movies, Tony Stark is consumed with his image. He puts out the carefree billionaire ego arguably to hide his emotions. He also uses his Iron Man armor as a protective shell to protect his physical self. In part one he was taken hostage, has shrapnel lodged in his chest, and suffers from Post Traumatic Stress Disorder after that. He doesn't have many close friends. His love interest, Pepper Potts, is the one emotionally closest to him, but he keeps even her at bay. In the Avengers movie, Stark is described as someone who doesn't play well with others. His character flaws list on, but in his essential core, he is a noble human being. He holds on to that and magnifies it during his most heroic moments. When he acts as billionaire Tony Stark or as the laser-shooting/I-have-the-latest-gadgets Iron Man, he is in his positions of power.
All my Google-powered research hasn't led me to an actual birth date for Tony Stark, but based on his personality traits, I'm almost 100% certain that, like me, he is a crab in the stars.