Thanks, Mal, for your insightful response.
Not long-winded at all! I enjoyed it very much.
Perhaps my choice of the word "habit" was wrong... but I do think you're talking about the same thing I'm talking about. I suppose I meant "habit" less as something that you do mindlessly because you've done it so many times, but more like being disciplined to know what is the right thing to do to acheive a goal.
Say, for working out, if you know you should be healthy and in good shape, you have to get into a habit of working out. That doesn't mean that every time I go to the gym, it's a mindless thing... hell, it takes mental effort just to get me INTO the gym several times a week. It's never easy!! But I try to stay in the habit, in the discipline, of doing what's necessary for my health, because I care about myself.
That's what I mean about relationships, too, and I think you agree... the willingness to work, to communicate, to spend meaningful time together, in order to stay healthy together. You said that your dad would call your mom every night, and later on they had dinner together and talked every night, and how they take vacations together. I guess that's what I mean by "good habits." Calling regularly, talking over dinner, communicating... making those actions such an integral part of your relationship that you stay as healthy and loving as possible, even when you don't feel like being that way.
But what do you think happened with your sister and her husband? Surely they didn't start out that way. What got in the way of their daily intimacy? Did they ever have it, and lost it? Or was it just never there? That was part of my OP... if love is being committed to working on a relationship, what happens to make love go away? Do all those little daily decisions that people make, that don't seem to matter at the time, build up to destroy the love that the couple began with?