Obviously married life is happier if the couple puts some effort in keeping it interesting. Being married is gonna suck if all you do is live with each other. You gotta live with each other. Go out, do new things, continue building upon your character. A lifelong relationship is miserable if two people stay exactly the same.
Sounds like a more specific version of "you get what you give". If you are willing to put out, work to make things...work, and compromise (not to the extent of being a doormat though) then a marriage should be balance and joyful. But different people have different personalities, so no matter who someone is paired up with - the success rate of it will depend partially on their own attitude
I would assume... It means... Eh, gonna be hard trying to explain my thoughts, but I'll try anyway. So I think it means that the happiest marriage would be one where each of the two people can be themselves, do what they love to do, think what they think and be generally free, while married to someone who they love and can be free together with. Not be all like: "I will do everything for you, like cutting my leg off so I couldn't escape" and the other partner being like: "Yeah, you do that, because otherwise I won't be happy!" It would mean they would be like one person, while being their own, individuals at the same time, know what I mean?