Deadpool has always shown a tendency to never take any situation seriously, stemming from his acknowledgement that he exists within a comic book universe. That makes him pretty unpopular with other superheroes in the Marvel Universe, so his adventures rarely involve others -- but one team-up with Wolverine and Captain America tragically set him up to lose everything.
During his lengthy run with the character, Gerry Duggan allowed Deadpool to build a family and an actual life. These developments helped Wade to connect with the heroes that had looked down upon him for many years. In fact, his wedding actually set a world record for most characters on a cover.
However, this time of prosperity for Deadpool eventually led to untold tragedy.
In the arc "The Good, the Bad, and the Ugly," co-written with Brian Posehn and drawn by Declan Shalvey, Wade became aware he was being followed by agents of Weapon Plus. Recognizing their shared past as super soldiers, Wade asked Wolverine and Captain America for aid, who both ignored his request for information and help.
Ultimately, all three heroes ended as captives of the program in North Korea, where the program head planned to use their DNA to create a new wave of super-soldiers. During escape and eventual rescue, it was Deadpool's discovery of a secret family that changed how both Wolverine and Captain America saw him.
Intent on freeing the other prisoners, Wade learned their families were being held captive at another camp. Included with these captives were Carmelita Camacho and their daughter, Eleanor. On the way, Wade played around with the idea of using a cardboard sign as an icebreaker with his new family. Instead, he found Carmelita at the top of a mass grave and he couldn't determine which body was his daughter's. This moment scarred Wade, but it also left an impression on Steve Rogers, who tried to console the heartbroken father.
Although it was later revealed that Eleanor survived, Captain America was changed by Wade's reaction in this moment. Gone was the irresponsible clown that Steve had known, as he recognized Wade's desire to help those around him and his own regret over past actions. He recognized Wade as a potential hero that desperately needed guidance and later, even invited him to serve as a member of the Uncanny Avengers. This invitation gave Deadpool the chance to become a real hero.
Wade's choice to become an Avenger was born out of his own hate for himself and his past failures. By becoming a hero, Wade was able to let others into his life, like Preston and her family, Shiklah and most importantly, Eleanor. He was on the path towards redemption for all his past mistakes. Wade believed he only needed to follow Captain America and he would one day become a hero. Unfortunately for Wade, Secret Empire was right around the corner.
Hydra Cap, using Wade's loyalty, tricked him into murdering Agent Coulson who had discovered Cap's true nature. Wade then lost the support of his friends and fellow heroes who had all come to trust him. From there, it was one bad choice after another as Wade was soon at the mercy of Stryfe, Cable's clone, who held the life of his daughter in his hand.
Deadpool's fall from grace began with the choice to believe Captain America when he said Wade could be more. Usually, this choice would have been the right one, but Wade made it at the worst possible moment. Every other bad thing that happened to Deadpool happened because he found himself completely alone once again. If Deadpool had never teamed up with Captain America, he would have never gained so much and lost it all so quickly.