[wideminded]

The best way to understand it is: if you look down the road and the guy in front of you seems to be pulling away from you 5 miles per hour faster than you're driving, and he looks down the road and sees the guy ahead of him pulling away -- also at five miles an hour faster than he's driving -- you cannot really come to any logical conclusion other than that the guy two cars ahead of you is pulling away at TEN miles an hour faster than you're driving.

Suppose you have a long highway with mile markers. The driver of Car A notices that every hour Car B passes 5 more mile markers than he does. The driver of Car B notices the same thing about Car C.

It does not follow logically that Car A will observe Car C passing ten more mile markers per hour unless the clock used by Car B goes at the same speed as that of Car A.

[FredZarguna]

The proper time of all observers is the same.