I don't disagree but I think there is more too it. Many forces working in tandem to push people into marriage.

Most simply, marriage is glorified by society. It's considered the ultimate declaration of love (within normal circumstances). Humans often feel the emotion so strongly and wish to express it. Society tells them this is how they should do it. It's also viewed as an achievement or a measure of love by many societies. You aren't committed (married)? Do you have cold feet? Do you not love them enough to marry them?

I may be viewed as one of those people for saying it, but there are a lot of pressures to conform to those around you. There is a certain life script - a series of things that those around you have done and expect you to do because... that's just what you do. You find someone, get married, have kids, etc. If you don't do these things or take too long to do these things you are defacto weird. You'll be slowly ostracized, gossiped about, avoided, suspected. You won't share the life experiences of those around it. It will be awkward. You won't fit in unless you manage to find enough atypical people to surround yourself with.

Also, most governments create a society where it is beneficial to marry, to encourage stable families and population growth. Certain circumstances can negate the benefits or even counter them, as some commenters have brought up, but in most cases married couples are given boons from the government.