When our Endorphins are medium to medium high we feel happy and our brain is receptive to new ideas, new connections and growth. If the Endorphins get too high, we can be too receptive and take in false ideas. If our Endorphins are low, we are likely to feel sad or bad and might find pain distracting. If our Endorphins get too low we can feel an absence of happiness, aka anhedonia.
Our Endorphin level is closely linked to having sufficient Dopamine, Norepinephrine and Epinephrine. If these go low, we often become (literally) un-happy. Even if these are at good levels, we need to be doing things we enjoy or thinking about good times to trigger this feeling.