Think a large part of the problem is what others have stated: marriage is some sort of social requirement. You grow up, go to school, get a job, get married and have kids. Or at least thats the expectation.
Many friends of mine (young 20s) are getting married. They get along with their SO but judging from the outside they are not enamoured in love like you'd think married couples should be.
If you don't get married and have kids you are seen as "odd". If, for some reason you take a little longer and are the last child in your family to get married there is a sense of relief eg "he/she FINALLY got married". The whole thing is crap to me.
I think you should only marry and settle with someone if you truly love them. If it doesn't click then move on. Don't try to mold them to you and how you want. Too many people are actively "seeking" spouse material. Friends are desparately using online dating service. I know one guy who has met up with at least a few dozen women on them and has a CHECKLIST of what he wants. You can't be like that, don't set up expectations because you may find that you will be greatly disappointed.
Let it happen if it happens but don't settle for less. Don't settle just to have someone around because your an attention whore or need comfort, security, the monetary benefits and someone to have kids with. Those are all the wrong reasons. If you don't find someone in your life that makes you giddy every time you see them, who doesn't make you slightly nervous when around (not from fear), who doesn't make you smile... then just CONTINUE BEING SINGLE.
I'll die single and lonely before I'll die after spending 50 years with someone who "I could get along with and we'd make good children".
/rant