Love, according to biological scientific research, then it is reaction of chemical substances within our brain. Those substances stimulate our so-called feeling: 'love' or 'attracted' to someone. Those substances tend to be produced many at first (beginning of love), but after 3 years the substances reduce, and that's why after a while of marriage the couples start to 'war' due to the fact that they start to see 'reality' or 'bad things' about their loved one. But what to do after that is another story.
...Anyway this is based on what I read, "Why man wants sex and woman needs love" by Allan and Barbara Pease, if I remember right.