Why all the negativity?

My life/living situation doesn't vary a great deal from day to day.  My job is the same, I have the same partner, the same children, the same chores around the house, similar things to do most days and most weeks, a similar financial situation most days and most weeks.  
So why does my emotional life sometimes vary so much from hour to hour/day to day/week to week?
Why do I sometimes feel great, alive, passionate and motivated?  And why do I sometimes feel sad, angry, irritable, frustrated, anxious?  When my external circumstances haven't especially changed?
I find that the way I'm feeling can potentially totally change my experience and perceptions and the way I react in my day.  I am working on being more present, less reactive, less defensive and more observant of the emotions that pass across my life rather than identifying with them.
When I notice these challenging emotions and negativity, and I have the luxury of time and space, I will work to release this with active meditation and visualisation; or walking in woodland/along the beach.
However, I also observe that sometimes I seem to want to hang on to the negativity.
To have inner peace, we need to let go of our desire to hold on to our pain.
Eckhart Tolle talks about our "pain bodies".  He says "All problems are illustions of the mind. Focus your attention on the now and tell me what problem you have at this moment.  I am not getting any answer because it is impossible to have a problem when you attention is fully in the Now.  A situation needs to be either dealt with or accepted.  Why make it into a problem?"