Before children, we'd go away for lots of romantic weekends, we'd stay to till 1am talking, go on long holidays. You'd be eachothers priority. The relationship was constantly being nurtured, and 'fed',
And then you have children, and they are now your priority. For a few years at least, the romantic weekends away disappear, the date nights are a battle to find time for, and although you can still have a lazy lie in, it's never together.
If you've got one of those partners that doesn't pull his weight, doesn't take an equal share if the parenting, that's when resentment comes in because you are doing more, more exhausted, and he doesn't get it. He won't get why you're crabby, why you don't have as much time to devote to him, or making yourself beautiful, because he just doesn't get the work involved. So there's resentment from you that you are carrying the family whilst he just swans along, and from him, that you weren't the women you were before. It's pretty relationship wrecking stuff.
If you have a good partner who shoulders an equal share, it's easier, but still challenging. Both of you come below the needs of your children, and by the time you deal with them and work, and life, there's not a lot of time or energy left. I have a fabulous partner and we are very happy and loved up still, but definitely feel the strain of not being able to devote the time to 'us' that we'd like.
It's hard to find the energy to nurture your relationship when you're wrung out at the end of the day, but when I have, I've felt better for it. Some time without phones of an evening, occasionally just having dinner the two of you, get in a babysitter, and just be nice to eachother. For a few years flashy shows of love are replaced with letting the other person have an extra hour in bed and then sneaking up to bring them a cup of tea and a cuddle whilst octonauts is on, rather than whisking them to Paris.