This came up whilst chatting with a few friends yesterday. The idea is that a lot of the qualities that make you a good partner overlap with those that make you a good person generally.
So if both partners are willing to work on the marriage/are not dicks, then being married is the one situation where you have to practice these things(e.g being less selfish, being more forgiving, thinking of others, being a supportive person etc etc) over and over again, sometimes several times a day.
You also learn a lot about your faults(e.g. holding on to petty grudges for ages) and realise that you have to work on them if you want a happy marriage.
Of course you can be all these things without being married, but the idea is that marriage forces you to work on these things daily, therefore making you better over time.
I'm not married, and my friends have been married for a couple of years at most.
I thought I'd throw the question out there to see what others thought.