rei’s hyperfixations

If we see it from Gon's perspective, would not that hurt?

Sure it is a decision made due to insecurities and an act of love because that's what he thought was the best for Gon. Killua being Killua, he never shares his emotions so most likely he would have hidden his sadness, act as if everything is okay and would tell Gon that he found something he wants to do and it's time to say goodbye.

Gon being Gon, would be surprised but would not stop Killua despite not wishing to part, but he would be missing him cause Killua's presence in Gon's life feels so natural to him. When Killua was wondering what he should do with his life Gon's was surprised cause his tone implied that it is a given that Killua should stick with him. Gon would definitely be sad and miss Killua and he would never know the reason he left, which can be considered hurtful and selfish.If Gon knew, he would be upset and even more sad at the knowledge that Killua doubts their friendship like that and sees himself as unworthy.

Yes, Gon can be selfish at times but also Killua's habit of hiding his emotions does not help their dynamic to improve either. It actually adds to the problem.

At the end of the day, they are two kids who mean the world to each other but they can get so lost in their emotions that they fail to be considerate of each other at times, and it's a perfect representation of relationships between teenagers, because I have not witnessed a single teen relationship where all parties involved have not hurt each other at least once. This does not necessarily mean that they don't care about each other, it just means they are too young to be in touch with their emotions and manage them properly.