San Francisco CNN Business  — 

For the last 35 years of his life, Paul Allen lived large and unapologetically.

He bought sports teams, airplanes, an island, and one of the world’s largest mega-yachts. He traveled the world, hung out with celebrities, SCUBA dived, and started a band. A billionaire philanthropist with varied hobbies and interests, he paid for expeditions to find lost WWII shipwrecks, funded brain disease research, and opened a museum dedicated to his idol, Jimi Hendrix.

He was an active businessman, too. Allen started multiple technology companies and invested early in AOL and Ticketmaster.

But Allen, who died Monday at age 65 from complications with non-Hodgkin’s lymphoma, was still best known as the co-founder of Microsoft, the company he started with Bill Gates in 1975 and left eight years later.

Allen’s stake in Microsoft funded his full and lavish life. An early brush with cancer, and his contentious relationship with Gates, pushed him to live like it wouldn’t last forever.

“I now understood that life was too short to spend it unhappily,” said Allen of the pivotal moment in his 2012 biography, “Idea Man.”

In 1982, Allen was already struggling with his place at Microsoft. The company had helped usher in the era of personal computers, creating the MS-DOS operating system that shipped on IBM systems.

The founders were fighting regularly, sometimes yelling at each other in front of employees. Allen attributed some of the issues to the fact that both he and Gates were generalists, and their partnership was more “intertwined” than the other famous tech founders of the day: Steve Jobs and Steve Wozniak.

But in his biography, Allen also claimed Gates was headstrong, aggressive, competitive and prone to shouting and anger.

Microsoft Co-founders Bill Gates and Paul Allen.

“Whenever we locked horns, I’d have to raise my intensity and my blood pressure to meet Bill’s, and it was taking a toll,” said Allen in his book. “Some people can vent their anger, take a breath, and let it go, but I wasn’t one of them. My sinking morale sapped my enthusiasm for my work, which in turn could precipitate Bill’s next attack.”

The partnership had also become unequal. Allen had the smaller end of a 60% / 40% equity split of Microsoft with Gates. He eventually agreed to an even smaller 64-36 share. As Microsoft began to get more press, Gates became the public face of the company. Soon, Allen said he felt his role was diminishing as he and Gates spoke less and less.

“Too angry and proud to make an emotional appeal, I never went in and told Bill, point-blank, ‘Some days working with you is like being in hell,’” wrote Allen.

Things hadn’t always been tense between the two. They met as kids at a private school in Seattle and bonded over their shared fascination with computers. Gates was in the seventh grade, Allen two years older than him. They stayed friends and in 1975, Allen convinced Gates to drop out of Harvard and start a company called Micro-Soft.

Seven years later, the friendship had eroded and Allen was already considering making a change. He wrote Gates a letter bemoaning the “destruction of both our friendship and our ability to work together.”

He was still at the company when he fell ill on a work trip to Europe with Gates that same year. He flew home to Seattle and was diagnosed with Hodgkin’s Disease. Allen was just 29 years old.

It wasn’t until later that year that the conflicts, the illness, and his growing need to find balance converged. Allen, who was undergoing treatment for his lymphoma, said he overheard Gates and Steve Ballmer talking about him one night at the office. They were unhappy with the fact that Allen was doing less work, and discussing ways to dilute his Microsoft shares.

Gates wrote a letter apologizing for the incident, according to Allen, but it was too late. He didn’t want his identity to become as tied up in Microsoft as Gates, or for the company to define him.

He resigned from Microsoft two months later, in February of 1983. His Hodgkin’s Disease went into remission and Allen started living a different kind of life, which would grow to include lavish spending and generous philanthropy.

Allen admitted to being bitter for a time after he left Microsoft, but said the feeling faded and his relationship with Gates improved. Freed from their toxic work interactions, they eventually became friends again. Allen later rejoined the Microsoft board and participated in Gates’ wedding.

“As the first person I ever partnered with, Paul set a standard that few other people could meet,” said Gates in a post remembering his friend on Tuesday. “He had a wide-ranging mind and a special talent for explaining complicated subjects in a simple way. Since I was lucky enough to know him from such a young age, I saw that before the rest of the world did.”