The most significant difference between lovers and friends is the privacy that they would need when they are together. There are things that only lovers do. Friends are usually there for their other friends at an emotional level but will not do things with each other physically. A lot of people are expected to have some friends, but they are usually allowed only to have one lover.
A lover is a person who the person can see himself or herself to spend the rest of his/her life with. You will treat your lover differently from how you would treat your friends. You will make your lover feel that he is the only one that you need and that he is the only one for you.