To answer your question directly, it's "the fifth thing",
but only special animal companions can leave an owner, not common, unique, or rare (wolves are in the common category). I realize I haven't posted all of this information yet -- been so busy at getting the system up and running that I just haven't yet, but I hope to do so today (Thursday).
Until then, let's break this summary down one line at a time...
First line: just indicates whether or not this companion is yours.
Second line: indication of general health (hit points)
Third line: indication of mood (based on personality, health, happiness, and hunger)
Fourth line: indication of hunger
Fifth line: indication of happiness
A companion can be quite content with life in general, but just in a pissy mood at that moment. While it may sulk and growl, the wolf is still well cared for and generally happy with the care its received, so the happiness rating (line five) is still high. You can help improve your companion's mood by keeping hunger low and happiness high, but each companion is different and it may just want to be sour for a while. Everyone has bad days, including companions.