Star Wars overhauled its lore regarding kyber crystals back in 2016, which changed the lightsaber crystal from simple energy sources to naturally Force-attuned objects. This lore had a lot of consequential changes, especially regarding Sith lightsabers, which previously utilized synthetic crystals.
During the first chapter of the 2017 comic Darth Vader, Emperor Palpatine explained that kyber crystals are "alive," before lecturing Darth Vader on how Sith blades are turned red. He explains that these blades must be corrupted by pain, and once a kyber crystal can no longer take this energy it turns crimson red, reflecting its user's rage. This process of imbuing a kyber crystal with negative Dark Side energy and pain is called "bleeding," which allows the Sith to have a connection with their lightsaber. Palpatine then forces Darth Vader to challenge a Jedi master so he can take and corrupt their lightsaber for himself.
Canonically, Kyber crystals are described as living things that possess a "collective consciousness." Their nature makes them naturally attuned to the Light Side of the Force. Their energies naturally fight off Dark Side influences, making "bleeding" an arduous process. When Darth Vader bled Jedi Master Kirak Infil'a's lightsaber, he nearly went insane from being subjected to the visions the crystal projected.
This change is drastically different from the depictions of Sith lightsabers in Star Wars Legends. Unlike the natural, colorless kyber crystals in canon, Legends kyber crystals were multicolored. This Sith used red synthetic crystals, which made their blades more powerful. Sith synthetic blades had to be imbued with Force energy for their use. These synthetic blades did not have the same connection Sith have with their blades in canon, as Palpatine simply gave Darth Vader and Darth Tyrannus red lightsabers in Legends.
The fan reaction to this change has been fairly divisive. Critics of this change claim that it eliminates the ability for Jedi to choose a crystal that fits their personality and energy on their own. Others are not a fan of the crystals being naturally imbued with the force, as it gives them an extra "magical" component. Those who like this new change feel it makes sense within the canon, as kyber crystal bleeding shows how the Dark Side is a distortion of the force. Supporters of this change also claim that it doesn't affect a wielder's personal choice, as each crystal needs to have a connection with its wielder.
"Bleeding" a lightsaber forces the kyber crystal to become as tortured as the person wielding it. This type of unnatural process fits the Sith mold, as the Dark Side corrupts the will of the living Force. The change in this type of lore makes lightsabers more than just a weapon -- they are an extension of their wielder, whether Dark or Light Side.