[eigenket]

Its probably mostly because you have an intuitive idea that there is some concept of "now" which is independent of the observer.

In special relativity this global "now" isn't a thing. It doesn't exist. There is no global now. Different observers who are in different places and/or moving at different speeds will describe different events as simultaneous.

In particular say we have an observer who sees an event A happening at time 0, and a second event (call it B) at time t and the distance between them is greater than c t. Then you can find observers who see A happening first, B happening first or the two happening at the same time. However all observers will agree that the distance between the events was greater than c times the time between them.

This seems like it would cause problems with causality, but it doesn't because we need the distance to be greater than c times the time, which means no lightspeed signal could get from A to B. If you allow ftl communication then this "escape" doesn't work anymore, and causality can be explicitly broken.