Unrequited love is the baggage of unresolved affairs, emotions and inaction that give our life color and drama, if we let it. On a day-to-day basis it can be constructive. I looked at you. You said something witty. You walked away. I wondered if you might be interested. I showed up next week... Consequently, both individuals were on their best behavior. Each had anticipation, and valued how they looked and acted more than usual.
In marriage, unrequited love is hell. It means that you both aren't close or trusting enough to work on the marriage. It means you both might be unfaithful, and definitely decided not to share what is meaningful. Unrequited love in a marriage is represented as 'you have your things and I have mine.' If you both have an agreement to care, you'll be fine. If not, consider leaving.
Unrequited love in life is unresolved past life involvements showing up in this life. Negatively, they look like rollercoaster relationships that can't be lived with, but can't be left alone. Many times drug or alcohol addictions show up with these relationships to double the problem. Constructive unrequited relationships may be with a person you mistreated who needs to settle, or someone you had interest in that agreed to meet you this life. Generally, those involved in both negative and constructive unrequited relationships need to soberly negotiate the details - past and current. They also need to build a supportive structure of trust with each other, or decide to set each other free.