I personally think sharing your whole life with anyone is hard, and I don't see unless I was a really placid person how it couldn't be.
Me and DH are different people, we have different wants, needs and ambitions. I have to accommodate his and he has to accommodate mine in every life decision. And we have changed as we have aged
Both of us are quite ambitious with our careers and have both had to make sacrifices for the others career. Equally we've both had to make sacrifices in our careers to stay with the other.
I don't find sharing space with people easy, I've always found living with my parents and friends in the past difficult. I've had to learn to share my space with my DH and he has me. I like my own space
There have been points where I haven't time for a relationship. You need to spend quality time with your SO to keep a relationship going and happy, and if there's work stress, Ill health, caring responsibilities, Dc then thats not easy. Keeping the relationship happy when there's no time for each other again is hard
I don't mean marriage is a slog when I say I've found it hard. We've not argued every day at any point and generally our marriage is very very happy, but that does take work for me and I can't envisage not finding parts of it hard
For me staying happy in general takes work, keeping motivated and enjoying my career, ensuring to spend time with friends and family and stay in touch with people I care about, eating well, excercising. All good things take work