My marriage is not perfect by any stretch (probably anyone who thinks what they have is perfect is a bit delusional). But I think we have a generally happy, solid, strong marriage. For us, I think what makes the difference is being really compatible with similar values and similar beliefs about how we want to live our lives and similar long-term life goals. Like we are very different personality-wise, we have very different career interests, but they are very compatible. I'm set in my ways, assertive, particular about things, he's much more relaxed, laidback, more spontaneous (we'd probably self destructive if both of us were as neurotic as I am). So each of us has traits that are really beneficial to the relationship overall, but we've learned when one of us has to step back and let the other take the lead on something. But we are very similar in terms of values, lifestyle, parenting styles, finances, very different careers (both fairly non-traditional ones) but very similar attitudes to work-life balance and beliefs about how to live our lives as a family. We also have very similar long term goals, like where we see ourselves in 10, 20, 30 years is very much in alignment. So like though we have day to day disagreements about minor things, the really big stuff we share a really similar perspective on and feel really passionately about that. Beyond that, we just genuinely like each other, we like spending time with each other, and we've been together 10 years now and have grown closer together over time rather than apart with age.
Also, I don't know whether it's a cause or an effect of having a really solid marriage, but we've been through some seriously tough shit together, both early on in our relationship (like 6 months in, and as recent as this past year). Not stuff like either of us did to hurt the other and cause problem, but due to things external to our relationship, like severe stress within the extended family, health issues, other traumas, etc. It was really hard and awful and stressful, but because we figured out how to support each other through that, we're really good at dealing with the day to day stress. I think because it didn't break us up, it made us even stronger and more of a solid source of support for each other. I think that definitely helps a lot, though our marriage is still far from perfect. We still want to kill each other sometimes!