Most people don't know this, but by-and-large relationships are actually quite predictable (as are successful marriages and divorce). Romantic relationships have relatively well defined stages, and how couples handle them -as well as the transitions between them -has a huge impact.
But I digress. There are three basic "brain atmospheres" throughout courtship and relationships, and they are divided by the primary hormones/chemicals/neurotransmitters present at those times (which means that while all these chemicals may be present, at different times some chemicals are much more prominent and present in the brain). Here's the story of what a brain looks like during love:
Stage #1: Testosterone/Estrogen -creating desire which leads individuals to (a) seek out others and (b) lust. Once two people meet and begin to interact, their "obsession" for the other person grows (and some of the chemicals from stage #2 begin to rise) and -kind of like a drug -people incessantly think about each other, miss each other when separated, obsess about the other person's fantastic qualities and are blind to pretty much everything else going on. This obsession leads to the kind of "can't eat, can't sleep, can't focus or get anything done" feeling that most people associate with passionate love. It lasts anywhere from a few weeks to a few months (depending on how often couples interact, talk, see each other, and so on) and it can be quite intense. Often times, this is known as the "honeymoon period," and is particularly salient around 60-90 days.
This is when you see two people who are super-affectionate and mushy, to the point that those around them are pretty much always thinking "get a room!" Or tell them to just chill out.
Transition #1: At about four months (though other life events, stress and pressure can trigger this transition earlier) the blind desire tends to wear off , whereupon certain illusions and delusions about the other person will shatter. At that point, differences become more apparent and couples often begin to argue about said differences. How they handle the realization that the other person isn't perfect or the man/woman of their dreams as well as the differences (and arguments) often determines whether or not relationships go beyond this stage.
Stage #2: Assuming the transition is handled well, the next couple of years (between 1.5-3 years into the relationship) tend to be where all the fun, happy-go-lucky, reward chemicals are present in the brain. Pheremones, norepenephrine seratonin and dopamine run rampant, making most experiences very rewarding ones and adding a kind of "fairytale happiness" to the relationship. In a certain sense, it's like a hazy stupor where everything the other person does is cute and funny. Their little habits are adorable, barring a particular and personal issue/aversion/disgust for whatever they're doing.
I've seen these couples. They've been together for a while, they've handled arguments so they seem to believe they've got the whole relationship thing covered. Yet, while something that they guy/gal does will annoy other people, their partner is oblivious, or just thinks it's adorable. I used to think, "what a perfect couple!" and wonder if there was some reason I don't have that. Couples at this stage make relationships seem a little too easy.
It's easy to believe in "happily ever after" during this stage, because everything does seem happy, and there isn't an end in sight... but then again, a lot of people get blindsided.
Transition #2: As these chemicals mellow out, the things that were once cute and funny may become annoying and frustrating so that now when a person's partner does the same things they used to do (for the past two years or so) it can grind on a person's patience. This isn't true about everything, but it tends to be true for some or many of the day-to-day habits people develop and have in going about their lives. How the frustrations are handled within each person and how they are expressed to the other (as well as how the other responds to the newly sour attitude about things that used to be seen as cute/funny/great) will often form a basis for expression and handling of each others' emotions (particularly the less happy ones). Listening and being empathetic versus accusing and being defensive may pave very different paths for a relationship. Assertive communication ("I feel..." instead of "You are...") communicates the internal process instead of an attempt to change or tear down the other person, which makes a world of difference.
Stage #3: Beyond the first three years lays long-term land, where oxytocin and vasopressin are the major players in an individual's brain-chemistry. These hormones are what create more attachment than twinkly-eyed, "take-me-now" type of attraction and can last many years or decades (think fondness/affection over attraction/desire). This is often a major part of the chemicals that are present during child-rearing (which, if you ask any parents, makes a lot of sense because it builds a sense of family and connectedness and when little ones are around there isn't as much time/resources for acting on intense desire... parents are often too exhausted). Let's be clear, though. These attachment chemicals don't mean that one's attraction or desire for their partner disappears.
Transition #3: Dealing with the increased demands of life (work/family/kids) and the "settling in" of the relationship sometimes leads couples to be more lazy. It's especially easy when there are so many excuses like kids (and difficulty finding a babysitter) that allow couples to say "we don't have time/money to go out." The result may be that fewer of the connections that existed throughout courtship and previously in the relationship are expressed through gifts, quality time spent together, touch, compliments/affirmations and actions conveying love. Balancing life and its many demands with a relationship simply doesn't get any easier, but by prioritizing and consistently finding -even small -ways to express care, desire and affection for one another makes a huge difference. It's tough, but possible.
There is also a transition (which is important to note) when children grow up and leave home, when couples now have much more time and freedom to explore and reconnect with one another.
While all of this can be interesting, one something to bear in mind is that seriously elevated levels of hormones and neurotransmitters isn't easy for the brain to maintain, so things would have to slow down and/or change at some point!
Additionally, these are very different stage of life/relationships, requiring a different way of handling one's life -juggling work, kids, paperwork, social time, intellectual pursuits, hobbies and passions and all kinds of other obligations to friends, family, government institutions becomes a whole lot more complicated -and perhaps this is the kind of adjustment the brain makes.
Really, who knows what comes first, change in brain chemicals or the events/milestones for relationships and families described... I certainly don't!