Ultimately for me it's about 2 things. GROWTH and The CONNECTION.  
I like to think (or, at least, HOPE) that each counterpart in such a dynamic comes to this point because they have recognized their deep core, are comfortable with it, and have a genuine interest in personal growth.  
And then - if you are lucky - you find a compatible counterpart who GETS you and is equally seeking to GROW.  
So, both seek that individual growth and TOGETHER they hopefully build a dynamic where their connection and the entire dynamic is also focused on it's own GROWTH.