Marriage - or a LTR - can be fun, boring, challenging, comfortable, infuriating, hard work, reassuring and everything in between!
It changes, because people change. You aren't the same every day, or week, or year. Your moods change, your wants, needs, interests - none of these things are static. So it's natural that your relationship will fluctuate and change as well. But if you are pulling in tandem then hopefully you accommodate the changes together. I've heard marriage described as a series of compromises and I'd agree (mostly!). It's rare that you'll find someone who will agree 100% with absolutely everything that you want, for a lifetime!
I stay married because I love my husband. He's extremely intelligent, witty, snores like a warthog, can occasionally be thoughtless, is sometimes utterly infuriating, capable, dependable and is very fair and honest. He makes me happy, even though there are times when I'm pissed off with him. We laugh together a lot - and that I think is part of the glue that holds us together. We have very similar humour, views on the world and a sense of the ridiculous. Been together 17 years and married for 14.
And that's marriage - you need something in common, whether that's children, or interests, or work or whatever and a desire to want to be with that person - to keep standing with them even when you are tired, and cross and broke or ill. It's the something in common and the desire to be together that powers you through the shitty bits. The sense that you are a team.