Ok, I haven't read any of the replies above, so sorry if I'm just repeating what others are saying, or if people have argued against this already or whatever, but if I read above I'm sure I'll start to think more, and never get round to replying.
I see marriage as a public declaration of the love and committment a couple feel for each other.
The marriage itself is no more than a bit of paper (though that bit of paper can be important for things legally), but it's a way of letting the world know how much you care for someone, and that you want to be with them for the rest of your life.
The strength of love is not added to or diminished by the marriage being there, or not, though I feel that if a relationship was going through a bad patch it would be harder to give up on it were you married.
All in all, I can't imagine not getting married one day. Though I wouldn't feel I needed to be married if I was just in a stable loving relationship, if I want to spend the rest of my life with someone then I want the whole world to know.
I think part of it is also the fact when you introduce your partner to people, if he (or she) is introduces as a boyfriend (girlfriend) or partner then people wonder hwo long you've been going out, how serious you are, so on, so forth.... and friend who've known you for years are likely to think 'how long is this one going to last' etc, etc. Whereas if they are your husband (wife) everyone immediately knows that you are committed to each other.
Anyway, does that make sense at all? It did in my head, but I don't think I've put it into words very well. All in all I think it can be summed up in my first sentance - it's a public declaration.