I agree with Anna, and what I think she's talking about is the same thing that makes a lot of arranged marriages work.
If you are 100% committed to your relationship then you work hard at loving someone, and at making yourself loveable. As time goes on, there are more reasons to love someone. Me and DH fell head over heels in love when we first met and he proposed a week later. 4.5 years later we have been married for almost 18 months and have 6 month old DS.
I still love him for all the reasons I fell in love with him, plus I love him for 'giving me' DS, and I love him for being a good father, and working hard to support us, and making me hot chocolate in the night when DS is having a feeding spurt.
There are lots of things he does that drive me up the wall, but we talk about things and never go to bed angry with each other no matter how much we might want to - so any frustrations and negative emotion are never around long enough to erode the love and respect that we have for each other.
I think people can be too quick to look for the bad in something, to say 'oh this doesn't feel quite as perfect as it did'. When you do that and choose not to work at resolving things, you are subconciously starting to look around at other options. Before you know where you are you have fallen out of love, and then it's very hard to get it back. This has happened to two of my aunts, and although they are still not divorced from their husbands, they all live very unhappy and unsatisfied lives because they feel shortchanged.
Falling back in love is possible. My Dad's parents were so so in love and have always been one of my model relationships. But they went through a very bad patch in their mid-fifities where my grandma really fell out of love with my grandpa. It took time and a lot of commitment for her to fall back in love with him, but they were of a generation that took their marriage vows very seriously.
God I've waffled on and on - sorry. It's something I feel very strongly about!