Why two points are always collinear?

Answer 1

Two points can always be connected by one, and only one, straight line segment.

In other words, two points always define a line.

And since they defined it, they're both on it.

Answer 2

two points are always collinear because there is only a single line which passes through these two points and also you can't any geometrical figure except a circle.

Also when u draw a circle through these points u will definitely get the diameter of the circle which passes through these points.