I think marriage is both depending on who you ask. Some people marry because they feel they have to (either out of religious and cultural beliefs, or because they think they will never find anyone else). Others get married because they absolutely feel in love and they believe that they have found their soul mate.
If someone has to drastically change in order for the marriage to happen or work, then it would be a very large compromise (for both people involved). If two people love each other exactly as they are, despite flaws and negative aspects, marriage would be less of a compromise.
Now, if you want to get into the legality of marriage licenses, that is a different slant altogether :P