Did not hear your show . . . (sorry!)
But to answer your question . . .
the best I have ever had "it" explained to me is--
a relationship lasts as long as the two partners or people are fulfilling each other's needs.
Once needs change, either the people in the relationship change to further meet the needs or the relationship is basically 'over', the choice being stay (miserable) or move yourself onto your next stage in life. Through my years I have found this applies to romances AND friendships. Family ties are a bit trickier (blood is indeed thicker than water), but still seem to follow this in principle.
The 'fun' is defining the 'needs'. They are not always obvious. For myself, realizing them (what my needs were at that time) has eased the transitions or breakups in my life.
That is 'what I think'.