Food for thought: Humans are invariably and innately self-serving.

I have held on to the belief that all humans, by nature, are inherently selfish. That everyone looks out for their own interest, for what suits them the most at any one point in time. And thus, because everyone has their own self-interest in mind, that makes people dispensable (to others) - meaning that if there's no reason to interact / maintain relationships / communicate with someone after they've served their usefulness, then we just move on.

For some, this may seem as a very cynical stand, but I subscribe to it, simply because i know it for myself to be true. The fact that people don't look for you unless they need / want something, and ignore you the rest of the time, or that aside from your perceived usefulness and role as the "last option", there is no other interaction going on between you and any other person. That you get discarded for a better/smarter/cheaper/faster/more attractive option. Basically, you're gotten rid of once you've outlived your usefulness.

Think about it. Companies do this all the time. Romantic relationships (sometimes) end in that way. Friendships have ended this way too (but more often than not they commonly attribute it to "drifting", "moving on to another phase of life". But the bottom line is there's no more reason to maintain the relationship).

So if something like this ever happens to you (or you unwittingly do it to someone else), remember, you're just looking out for yourself. Its not a matter of this being a good / bad thing (it really depends how you spin it). But the point is that it happens. ALL. THE. TIME. and you'd be naive to think that it doesn't.