In other words and in addition to the previous post, initially when you start increasing Vd from 0 the transistor is in triode and works as a switch, just imagine short and then of course source voltage will follow the drain voltage. After the transistor enters in saturation, the circuit becoms a source follower and the source voltage remains fixed by the gate voltage and does not depend on the drain voltage.
Then, I assume you understand why those things are happening - transistor in general is a symetrical device i.e. its structure at the source is the same as at the drain. When your drain is low, say 0v both Vgs and Vgd are big enough to support a channel at the source and drain. However, when you start increasing the drain, the Vgd decreases and at a certain point (about Vd=5v according to your sims) the gate voltage will be just a Vth higher than the drain voltage. A bit further on and the gate will not be high enough with respect to drain to support a channel at the drain side and the transistor goes in saturation, the channel is pinched-off. This happens at a certain source voltage (remember the source has been following the drain up to now) which defines a Vgs which in turn corresponds to the drain current which is now constant to a first order and does not depend on what happens at the drain. And in this case of course, the source will not change, the circuit is a source follower with a fixed gate voltage.