The secret to a happy marriage is to find the right one, then marry her (in that order). Most people settle for less, thinking it's the best they can do. Maybe they marry one that just needs a "little bit of tweaking". That doesn't seem to work well for most. Always amazes me when I ask a person what attracted them to the other in the first place, one of the frequent responses is "they liked me". Doesn't say much for self-esteem, does it?
Also, considering the human brain isn't done "growing" until age 26 or so, most marriages with one or both under 26 end in divorce. They say stuff like "we don't have anything in common" and "we've grown apart"; and "he or she isn't the same person I married". And they are right. Sometimes it's "I thought if we had another child we would be close again, like when our first child was born". Sad.
If you can't find the right person it's best to wait until you're 96 years old, and then ONLY get married if you "have to".