In the past lots of people were forced into marriage by their sheer inability to survive alone. Women would have a hell of a hard time supporting themselves, and the men were widely incapable of doing basic things like dressing themselves or making a sandwich. That's a very strong bond; you see it still with old couples who despise each other, but have to stick together because the man doesn't know how to run a washing machine and the woman doesn't know how to drive.
Of course I think of people now as being pretty independent, but a lot of them are still totally incompetent. I'm amazed how many men still can't do basic cleaning or cooking (I'm sure they quite willfully keep themselves ignorant because they don't want to have to do that stuff). Lots of girls still couldn't get a decent job if they had to, because they were Communications majors or something and have zero skills. And of course all those suburban princess girls are just completely worthless all around, they can neither cook nor clean nor do any decent work. They only thing they know how to do is put on make up, wait tables, and give blow jobs.
Relationships without artificial glue are very difficult to sustain. As I've already mentioned, one form of strong glue is if you're incompetent in some way and simply can't live apart (or societal repression as still exists in much of the world). Another form of strong glue is children. Even if you aren't fully in the mode of "we hate each other but we'll stay together for the children", children are still strong glue in that it simply gives you something that you both care about and have to take care of all the time; it gives you a common mission, and it also gives you a focus of grief that's outside of each other.
Another form of strong glue is being a loser. By being a loser I mean the fear of being single or the belief that you can't do better, or inexperience dating. If you marry your highschool sweetheart, you're terrified of having to get back out there and date. You believe maybe this is the only person you can ever find, you're afraid of what single life would be like. That's a strong glue.
Modern society celebrates independence and consciously choosing a mate - that is, being in a relationship without all that glue. But with none of that glue your only reason to be together is because you think the life with this person is better than any other life you could have, and that is a very hard bar to meet all the time.
So, my advice to someone who wants to have a strong relationship : 1. make yourself incompetent, if you're a man never learn how to cook or clean, have your mom always pick your outfits for you. 2. marry the first girl you date or have sex. 3. have kids right away. 4. don't let your wife go to college or learn any skills.