Why Does Colon Cancer Come Back After Chemo?

I'm having my second go round for chemotherapy...my colon cancer metastisized to my liver so I'm back in the chair again. I am angry (not so much anymore but it's still there, I am sad, crying difficult to discuss, not understanding the whys or how this is returning. I normally stay away from groups because they make me feel more down then anything but decided to see if this one will help. My hair has thinned out so much every time I comb it it becomes more than and painful to me. I am working on my diet juicing and more veggies..this is my first hookup week two weeks ago I had a rough reaction so this week they slowed the medicine up. My question i researched is why does it come back after the harsh treatments provided is it based on the stage of the cancer one was diagnosed with? Then the weight gain comes back the craving come back which I have eliminate a lot of the sugar cravings I once had. Like myself there is someone out there that may feel moreso like this is a venting session and you may or may not obtain some very good insight. We all strive to be better. Blessings to you all