There are state laws, religious laws and nature laws. Love belongs to the law of nature which should be the same everywhere. Religious laws differ from society to society whereas state laws differ from place to place.
You can prove your love to your beloved one without any piece of paper but for others to accept that you two have the "natural bond," you need to present evidence -- which, in Islam, is the Nikah document.
so others are blind and need a piece of paper to really believe that you're in love?
so ultimately, all us humans want is to be accepted by others? if we don't provide them with evidence, then they won't accept us?
marriage, nikkah or whatever is just to seal the deal...and only has to do with the state/religion. it also gives couples a sense of security.