You have to work hard in a long term relationship, because people are human, have flaws and take each other for granted. Relationships go through peaks and troughs, as do friendships, family relationships etc.
To 'work' at something is to prioritise it every so often, to make the effort to make time for each other (when there are a million and one other things that need doing), things that don't always happen when you've been with someone for a long time. Sometimes I look at other people, couples who've only just got together and think, where does that magic, that insatiable desire to rip each others clothes off go, staring into each others eyes? That passion, that interest, that love for each other needs reminding every so often. Maybe it's the term 'work' that sounds like 'hard work'. I think it's more just a gentle nudge every so often. It's difficult to not take someone for granted, when they're always there, have been for years, when you know how they think. I have to tell myself not to take DH for granted.
Mind you, thanks for this thread, because it gives me a kick up the arse to tell DH I love him and appreciate him. And yes, I do take him for granted, so yes, our relationship does always need 'work'.