e = mh2

Where m "mood" and h is the constant for "health".

I found a very well phrased summary of why it's hard to have, make, or keep friends when you're dealing with an energy sucking illness. It's on Abide, a blog by sbpoet. Here's a small sample of her post.
And for the person who is ill, friends are seen in a new light. When energy is severely limited, one cannot afford relationships that exhaust and do not renew -- even if these relationships were enjoyable when energy was cheap.

Of course a poet would be able to turn this phrase so well. I remember back when my energy was cheap too. An image instantly springs to my mind of a teen standing in front of the refrigerator, door wide open, while he ponders - at length - whether he wants a glass of orange juice or lemonade. This was how I was with my energy before.

It's not an exact analogy though because one's own personal energy is not like power from the electric company. Carefully conserving it in my 20s, always having been sure to close the door to the fridge while I made the moment to moment daily equivalents of beverage decisions, would not have resulted in my having any more at my disposal now.

Still, I sometimes feel like I squandered what I had.