Love is a feeling or emotion, whereas marriage is more of a ceremonial event to formalize a change in someone's civil status from being single to being married. Marriage is more equated to the commitment and entering into a legal partnership. Romantic love is akin to some mixture of friendship, passion, and sexual desire.
Some people get married, but they are not in love with each other romantically. Love could also signify a strong bond between a parent and child, siblings or other family members, friends, a beloved pet or animal, or it could mean the religious type of love, such as love for God or church.
Love is an emotion and a feeling. Anything can be loved. A person can love a dog, love a song, or love a season. Love itself is pure, but doesn't necessarily have to be a commitment. Marriage is an act and commitment, which comes from love of two people.
Though love can occur between two people without this act, in a way, marriage solidifies the love and makes it legal in the eyes of the law. With marriage comes many legal and financial benefits, including saving on healthcare, filling jointly on taxes, and the ability to transfer benefits to one another.