...and expectations fall dramatically flat after the big day?
As many a spouse will testify, marriage requires work. The exact nature depends on the couple but usually involves a fluid and constantly adapting strategy involving strengthening your foundations, pursuing the art of compromise, and evolving your expectations - both individually and as a couple.
When seeking success, you wouldn't approach any other aspect in your life - from your career and friendships to health and fitness - with complacency, so why rest on your laurels when it comes to love?

Marriage will make things easier
The idea that as soon as you're married, everything will magically 'fall into place' and become perfect is as misplaced as it is misleading. The strengthening of your bond borne from the extra commitment will undoubtedly make you more determined to invest into your relationship and tackle any problems as a unit, but the flipside of that is that your lives are so inextricably linked that disentangling your needs, desires and emotions can prove equally as problematic.

We'll argue less
Contrary to some opinions, conflict is a healthy component of a marriage because it means that you're honestly communicating your feelings and actively trying to find a resolution. Emotional intimacy comes from truth-telling, however hurtful the process may be; the trick is to ensure that arguments are the path to peaceful resolution and greater understanding, not used to score points, dredge up resentments, deliberately cause hurt, or initiate power plays - all of which will have a negative impact on your marriage.
Over time, you'll develop better dialogue, intuition and dispute-resolution skills so that you argue less. As long as you're able to analyse why the argument happened and how to pre-empt and dissipate similar rows while honestly communicating with each other at every stage, you'll emerge all the better for your clashes.

Getting married will solve all our relationship problems
Every couple, whether married or not, has their issues. Human beings are a complex breed and problems will inevitably arise. Marriage isn't a blanket cure for all problems, and in fact can perversely heighten any pre-existing fractures, such is the spotlight it throws on personal vulnerabilities.
The point is that marriage forces couples to refocus and reinvest in their relationships, which can improve problems as long as it's a continual and positive process. Rather than criticising or blaming each other for shortfalls, work together to right them and achieve harmony, by consciously being better together.

I can change my spouse
Marriage doesn't tend to change people. It may make you want to consult with each other more/spend more time together/demonstrate your love in new ways... but, ultimately, the fundamentals of who we are as a person are unlikely to alter. Remember, you married them for everything they are, and the process won't automatically make them change, or even want to.

If marriage doesn't meet my expectations, we've failed
There will be moments - whether fleeting or protracted - when you feel as if you've failed in your marriage. If things stagnate sexually, if you feel unable to meet your spouse's emotional needs, if you encounter money difficulties, health complaints, bereavement... it can all have a drastic impact on your relationship, your ability to cope or connect, and your idea of what makes a successful marriage.

My spouse is responsible for my wellbeing
The only person responsible for your happiness is you. Others can amplify or lessen it depending on their behaviour and role in your life but, ultimately, it's all down to you. You complete you; and your deficits are your own. It's the difference between sharing a life and co-dependency - one is bolstering, the other dysfunctional.
Placing your unwavering faith in your love is one of the joys of marriage, but allowing your wellbeing to be solely defined by one other person is both dangerous and an unreal expectation, not to mention a huge amount of pressure to place on your spouse.

