why?

I often pause to count my blessings and I guess when I tally up the pluses and minuses of my life I end up with a positive so why do I feel negative sometimes? why do we so often desire what we don't have?
Over the past weeks i have been busy working and it has been good, I have enjoyed the work and I have enjoyed being a worker again, it feels good but I still have that place deep inside me that is hurting and telling me that I'm not a good person. I know this is all part of my illness and I have employed a technique given to me by a wonderful friend which involves visualising two wolves inside me, a good wolf (positive stuff) and a bad wolf (negative stuff) I have tried to keep feeding the good wolf and starving the bad one. I haven't explained that very well but believe me it has helped me so much.