Once people see each other as granted they stop trying to please each other, stop trying to be nice, and more and more we are able to see what the person really is.
There is also the fact that sometimes the person is not willing to do what you wish at the bed. Then there is the dilemma you love the person, you are married, already had a conversation about sex with your partner, and haven't changed, you also don't want to have problems finding someone who wants only a nightstand also doesn't want to waste time on dates, don't want divorce, the only option left is provider.