Cancer men and co-dependence

A couple of Cancer men I know both got out of long term relationships recently, and both said one of the reasons why the relationship didn't work is because they were co-dependent with the ex. What does co-dependence mean? And why is this such a common theme in Cancer relationships?

I've never been co-dependant on a gf, but like all crabs I get very attached to the relationship, even if it's not working as it should. I just think that cancers in general are relationships people. And especially the men love to be in love. From there to became co-dependant on a partner it's only a small step!