Dating a Cancer man is like dating a moody woman, but one who doesn't express what they feel, instead running away sideways and attacking by abandoning and passive-aggressively disappearing instead of talking about how they feel and what’s going on. Which can be the ultimate in hurtful. Cancer men particularly can be the most insidious, because they pull the "suddenly throw you away" thing like you’re just a piece of trash, when really they are the trash by doing this to people. The Italian mafia are probably all Cancer men. Pretend they like you so they can lead you into an elevator and kill you. Then bury your body, so no one says or knows who did it because they all sneaked off sideways into the night and disappeared forever.
Cancer men are the "mothers" of the zodiac. They need to be nurtured and need to nurture others, which often causes the creation of a strong bond with their mother, making them appear as a stereotypical "mama's boy". Though this trait is also the reason most cancer men make a great family man.
It is in the crabs nature to distrust just about anything and anyone around. That is what helps him survive in life. The real question isn't whether or not your man is insecure, the real question is 'how thick is his shell'.