Harry Potter was used to the Dursleys bullying and mistreating him, but didn't stand up for himself — despite immediately standing up against Draco Malfoy when they first met in Harry Potter and the Sorcerer's Stone. For the most part of the main Wizarding World film series, Harry and Draco found themselves on opposite sides, competing against each other. While the young Malfoy was prone to altercations due to his arrogance, the Boy Who Lived had an aversion to confrontations having endured years of abuse from his foster family.
In order to keep him safe, Harry Potter was left under the care of his maternal aunt, Petunia, and her family, where he spent the first 11 years of his life until he received his acceptance letter to Hogwarts. During this time, he endured near-constant physical and emotional abuse from his family. In order to cope, he kept his head down and his lips sealed. This didn't stop the Dursleys from harassing him, but it also didn't make his predicament worse. As someone who had depended on this coping mechanism for dealing with bullies, it's interesting that he didn't hesitate to face off with Draco.
Unbeknownst to him, Harry was a celebrity in the wizarding world, so classmates were already anticipating his arrival at Hogwarts in Harry Potter and The Sorcerer's Stone. That included Draco. To be fair to Lucius and Narcissa's son, he did attempt to make friends with the Boy Who Lived — perhaps because he knew of Harry's social status — introducing himself as well as his two sidekicks, Vincent Crabbe and Gregory Goyle. Before Harry could even respond, he witnessed how Draco looked down on and humiliated Ron in front of others. Harry decided, based on that alone, that he didn't want to be friends with Draco. Subtly, this scene also explains why Harry had no issues standing up against Draco despite avoiding conflict with the Dursleys: while he could endure years of abuse from the Dursleys, he's extremely protective of others. Seeing Draco and his cronies mistreat Ron activated Harry's protective instincts.
The exchange with Malfoy also gave Harry the extra motivation to stand up against Draco knowing that they were peers. Both were first-year students coming into Hogwarts in the same year. From Harry's perspective, it was an even playing field — he didn't need to appease Malfoy. The Dursleys, however, had considerable power over Harry. Since he was a minor living in their home, he needed to do whatever he could to keep his situation tolerable. Being away at Hogwarts gave Harry true freedom for the first time: having the opportunity to exist on his own was a privilege that allowed him to finally live by his own values.
After being abused and isolated for so long, Harry really treasured the genuine relationships he made in Hogwarts. Time and again, he proved that he wouldn't hesitate to put himself in danger just for the sake of those people. In the end, while Draco's bad behavior was mostly caused by the way he was brought up, taking Ron's side in Harry Potter and the Sorcerer's Stone proved to be one of the best decisions Harry ever made, considering how the Weasleys became such an integral part of his life moving forward.
