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?
"That is why..." We have absolutely no way of knowing what is going on here because "that" which is being referred to isn't here. For all we know the person who quoted it could have been looking in the can after dropping a particularly glorious turd and remarked, That is why I say that the real happiness of married life depends largely upon the personal character that is put into it.