The thing is, it takes an evolved Cancer to understand WHY they are going thru the phases they go through. Meaning, taking a look from the outside in, the way we see them. Until they reach that level of self understanding, theyre blindly riding their own emotional waves as much as the person that's with them. So if they can't see it, but the person with them can, and has enough love and patience and determination to keep being with the Cancer, its up to them to hold a mirror and help them out.
You can't learn how to be a better person without interacting with others. Cancers don't see the world outside of their shell unless you shove it in their face (kindly, of course! Lol).
So StarMooney's example is perfect... It's up to the Cancer to express what it is they need, and when they need it. Myself as an earth sign, I'm going to give you the same type of loving every single day...that's the stability in me. If you need something different, as a Cancer, during one of your phases, you need to speak up. But like I said, certain times of the month, after a few months, it gets easier to read...although usually for me, too late lol. If I could get AHEAD of the mood swing, that would be treetrunkin amazing! Lol.