The moment you start taking your relationship for granted is the moment your relationship is doomed

Every new relationship starts off with hope and possibility. As you're just getting to know your partner, you're finding out their quirks, while they're trying to figure out yours, too; and while you're sure to step on a few land mines along the way, the hunt to find them is part of the adventure.
They don't call it "New Relationship Energy" for nothin'.

Eventually, if the relationship continues, it starts to sink into a sort of routine. You begin to make plans surrounding the other person's schedule—not just time-wise, but emotion-wise: you know if they greet you with a capturing smile, it's time to smile back; if they grin briefly then turn away, it's time to give them some personal space; if they give you an evil grin with a hint of mirth and a sparkle in their eyes...and you happen to be a sub-type...it's time to run.

You begin to understand the other person—what makes them tick, what shapes them, how everything in their history has led them up to who they are today.
And slowly, as you sink comfortably into this new routine, a new sense of security sinks in.

Which is wonderful. It may not be as exciting or titillating as the New Relationship Energy was, but it's got its own magic and appeal. There's the knowledge that you've gotten to know each other well enough, and come to care for each other deep enough, to be fairly certain that the other person will be there for you through the thick. The more memories you make together, the more certain you're likely to be.

This assurance feels great.
It can also be dangerous if you start taking your partner for granted.

You start making assumptions about their needs, their wants, their desires...what is a necessity for them in the relationship, and what your partner can live without.
Things you used to do for them, you no longer do. Sweet words of love you used to tell them, you no longer bother to say. Or maybe worse, you start saying things to them you wouldn't dream of saying to a stranger on the street.

Why are you being so rude to a person you claim to care about?

Little favors you used to do for them to make their lives better, you don't bother offering anymore...because why should you? Your partner isn't doing you favors anymore. They're just...doing what they've always done, and will always do.
Except there is no always. Not in any relationship.

I don't care what kind of relationship you have—D/s, M/s, O/p, or pure vanilla. Don't care, it's still true: people need to feel appreciated.
The moment your partner starts feeling like they are not appreciated? THAT is the moment a crack forms in the foundation of your relationship. Doesn't matter how old the relationship is, or what you've been through together, either.
A person who feels taken for granted is a person who starts slipping away.

I've seen marriages fall apart because of this; Master/slave dynamics crumble like wet sand, often leaving one partner staring at the receding back of the other, wondering what the hell just happened.
I thought we were fine, I sometimes hear. I thought we would last.
They stopped seeing their partner as a blessing, and started seeing them as something they would forever have—and deserve—no matter what.

People who've been in long term relationships can tell you, things don't work that way.

Let me be clear, I'm not talking about 'the-chips-are-down-extenuating-circumstances' situations here. When one of you gets sick, or loses a job, or suffers a death in the family...hell yes, you should be able to rely on your partner to be there for you. (At least, as much as they can be.)

But to take your partner's help for granted, and not consider once what their help is costing them? That rings out the cloister bell of doom right there.

Always remember, your partner's contributions to the relationship need to be acknowledged. They need to know they hold value in your life.

(Unless they don't. In which case, your relationship is doomed anyway.)