Motivation for staying in a situation of unrequited love is also an important factor when writing. Are they emotionally immature and don't realize their feelings, leading to anger/jealousy/a sense of ownership over another? Do they feel as though their feelings are wrong and are fighting themselves using said feelings or societal expectations? Do they value their friendship and occasionally have bursts of romantic thoughts that ebb and flow throughout the decades?

Where they place their feelings as rank of importance in their life will guide a lot of action, and of course, your writing.