If you make a conscious effort to guide/steer your attention toward thoughts and subjects that feel good, your default emotional setting rises.
We have an equilibrium that we consistently return to as our attention relaxes. This equilibrium is essentially an emotional state that constitutes our "average" mood.
One's average mood drifts up and down due to the accumulation of experience, as well as other factors. When our average mood is good, we might refer to ourselves as having a good stretch, and when our average mood is unpleasant, we would say that we are having a bad stretch, or a difficult or challenging time.
However, by monitoring one's emotional state, one can CHOOSE to entertain thoughts that feel good. You must take responsibility for your state of mind, and deliberately steer your thoughts and your attention in a direction that feels better.
I have noticed that by deliberately searching for better feeling thoughts, I am able to raise my equilibrium emotional setting - my average mood. When I do this, I can experience joy and happiness which is not being triggered by any particular event or circumstance or thought. It is a general peace and appreciation that comes through in waves.
One way of looking at this is that my average mood has gone up as a result of consciously and deliberately and consistently steering my attention toward good-feeling thoughts and ideas. And since my average mood is my default emotional setting (i.e. the one I return to when my attention is not on anything in particular), I am arriving at this peaceful state automatically, without trying.
And a little voice inside of me is saying, "Why have I never heard of this before???"