There were three unspoken rules for the Marauders. No one but James can flirt with Lily; Nobody talks about the furry little problem in public; Never- NEVER touch Remus’ wand.
The only times Remus ever snapped was when somebody messed with his wand in any way. Nobody really knew why he reacted so strongly. Wands were important and special, but he was just a little too intense about his own.
The reason was a secret that he kept to his dying day, and after. That wand was special. He cared about that wand more than practically anything because of where it had come from. It was the only wand The Whomping Willow ever allowed to be made from it. It was never just a wand, it was a promise, a beautiful vision of love.

There was a hole in all the Marauders at some point of another. Peter filled his with friends, which he had never had, though they didn’t satisfy him for long. James filled his with Lily, and Sirius filled his with James. Remus, though, didn’t find such an easy fix. His friends made his life bearable, but he still wanted for more- though he couldn’t for the life of him figure out what. Then one early morning he awoke from his wolf form in the place that was his prison. His friends had left him with his wish of being alone, and he was just that, watching the strange colors dance around the sky.
Then he heard him. The voice that sounded like leaves rustling and wind blowing, soft and warm and hesitant, so perfect and so whispered that he thought he must have imagined it. But time there showed that it was no fantasy. The tree was talking to him, sharing with him what he knew of their strange world, promising him the stars, and being assured that he wasn’t truly hated. They thought the same, and argued shared views that made the other think. They spoke as equals. It wasn’t long until feelings arose. Tree or not, his love and his mind were human enough, and his heart, though made of wood and pumping sap instead of blood, was more real and truer than much he knew.
He was nervous, he was shy, he was downright terrified, but he had no reason. Whompy felt the same, and admitted it before his words were even out. And from then on they were together exactly how they wanted to be, secrecy a small price to pay. They had started something new. They had started something perfect.

Stole the Whomping Willow's back story

The Whomping Willow had a sad existence, lonely and cold and silent. The only times he was happy, was when he was with Remus or when he was writing music. Dumbledore noticed this one day, and talked to the tree the way it would only allow a few to do, and they devised a plan.
They found a Squib who had gotten into a lot of trouble with the Ministry, and he was used as a surrogate, placing Whompy into him, leaving his tree body behind to mindless and violent. With his new body, he toured, playing music, but kept the one thing that could make his life complete at the front of his mind- the love of his life. He would find his Remus again.