Happiness and comfort.

One can often lead to the other, though both cannot always be sustained in unison for extended periods.

What does an overabundance of comfort lead to?

A soft body.  A blunt mind.  Languid thoughts.

Think.

When is the last time you accomplished anything while in a state of comfort?

If necessity is the mother of invention, then comfort must only beget laziness in time.

Do not allow contentment to become smugness, nor rest become sloth.

Get uncomfortable!