The Valleys of Relationships

So, why is it that we get complacent in our relationships?  Why can't the "honeymoon phase" last longer than a honeymoon?  It's so frustrating, and a cause for many a disagreement I am sure.  Of course the obsessively absorbed stage can't last forever either because let's face it, nothing would get done.  But there has to be a happy medium in between manic and complacent.  If I could find it, or knew where to get the answer I could make millions happy.  We all know that relationships like life take work.  And all the "experts" say that communication is the key to success.  But seriously--even these "experts" must have days where they have been busting their butt, and their spouse/significant other/better half (whatever you call it) comes home and has had an equally butt busting day.  Do you really feel like talking?   Or does every word out of their mouth just serve to infuriate you?  Because you are tired and frustrated and feel taken for granted.  How do the "experts" deal with that.  And while I am talking about experts, how does one become an expert?  Is there a degree in relationship expertise that is being offered at our local colleges that we know nothing about?  Because I would sign up for that.  And before anyone gets any ideas about my life of course I am speaking in the hypothetical when using specific examples.
But it's not just spousal relationships.  Friendships take a similar turn.  You talk, text or get together often when you are in the "getting to know each other" phase.  But soon those days of talking and getting together start happening less and less frequently.  There's always the excuse that "we are so busy".  But in most cases we have the same lives that we did in the beginning of the friendship.  Why was it so convenient to get together then?  Was it because we didn't know each other that well and didn't want to disappoint?  So now that we do know each other its okay to disappoint each other?  Seems to me like that logic is backwards.  If we don't really know each other that well, why are we worried about hurting each others feelings?  It must be because some level of understanding occurs with familiarity.  We understand that we are busy, tired, insert the appropriate excuse here. 
This logic also applies to our relationships with ourselves.  Why is it we become complacent with exercise once we reach our goal weight?  Or we are complacent with even starting an exercise routine because we are comfortable with being "this way".  Same thing with diets.  We watch what we eat and avoid all the yummy things in life to what?  Get to our "goal weight" and what we do?  Reward ourselves with Food!!!  How insane is that?  Its the search for the happy medium between manic and complacent.  Know what I mean?
They say an object in motion will stay in motion. But aren't relationships a moving thing?  So, then why do they so often get stuck?  Or worse, seem to be on a perpetual roller coaster?  I don't know about you, but I HATE roller coasters.  The feeling of my stomach in my throat accompanied by palpitations is not something I would pay for.  But yet, that is kind of how you feel when you fall in love isn't it?  Sick but excited at the same time? 
The peaks are what everyone strives to maintain, and the valleys are the place you can't wait to get out of.  Why can't there be a flat plane?  Of course a flat plane of  peaks. Why the peaks and valleys at all?  Good question to ponder.........