Why marry...
Anyone who doesn't care about the vows he/she gives shouldn't marry. If you
promise something, you should mean it and stick to it.
I see marriage as a bond that is stronger than others, in which I know I can trust my spouse and in which we gave promises to each other- not just the ones during the ceremony but also the ones in privacy.
To the ouside world it also means that you are taken and dedicated to one person only.
I've recently moved to the States and the divorce statistics is horrifying. I also think most Americans rush into marriage- after having known a person for a couple of weeks or months.
In Europe couples wait longer to get married (in some cases tooo long), anywhere between a year and 5-6-7.
Ultimately, nobody is forcing you to marry. It is up to you. If you and your partner cannot agree upon what you want and what you expect of your relationship but like being involved- well, you do not have to marry!