I wouldn't necessarily go as far as to say the sun is intelligent. It is hard to put in words, but I believe magic has this sort of trickle-down effect. The source of Aurelion Sol's creation is presumably the source of it all (to our knowledge). He then uses his imbued powers to create something inconceivably large and powerful like a star/sun. This sun is sending out all kinds of waves and rays and energies that are somewhat mysterious (does Aurelion even know what they are capable of?).
From there, the magical energies trickle-down to Runeterra. I imagine, just like our universe, these energies are useless without technology or other tools to harness them into a productive/destructive force. The Ascension is that tool. I would compare it to how we need solar panels in order to convert the solar energy (that exists independently of our existence) into electricity, which is useful for us. Now, plants can take this solar energy and have natural biological processes that allow them to convert it directly into sugar (photosynthesis). Reptiles use this solar energy to maintain the correct body temperatures. Even humans naturally absorb vitamin D directly from solar energy.
I am imagining Runeterra has similar mechanisms, but in a more fantastical way. For example, that magical energy may be funneled into the Sun Disk and then requires a suitable candidate to absorb the energy. This individual would maybe need to maintain a particular vibration or personal energy that manifests from everyone being a piece of the Source. Some individuals have probably become aware of their personal energies and exercised them in a way or may be unaware of their personal energy and exercised it anyway. I would compare this to having the correct wires to plug into the outlet. So, while Azir may have the vibrations of a copper wire and absorbs the energy of the Sun Disk in a clean, productive way, Xerath is more like an aluminum fork that got forced into the outlet. He will still conduct the current, but it will be a destructive force.
That may be a poor comparison, but that is just how I rationalize the idea of magic. As Sharjo mentioned, Soraka (and maybe others) can communicate with the stars, so there are probably several different types of energy and magic pulsing out from the Sun, and individuals/civilizations may need to create the proper tools to harness those energies. Soraka probably has natural mechanisms that allow her to have these powers that cannot be understood by others. I would compare this to birds who have internal systems for interacting with Earth's magnetic field, but most humans do not have this "power". However, we created the compass, which operates is a similar way and serves a similar function. I just is not something we have innately (or at least to the same degree as migratory birds).
"This sun has been of its axis, it has been driving me crazy for aeons now..."
