replacement

can someone really replace another? in a job I mean.
sooo easy one in our office makes space for another. and that's it. bringing along different abilities both academically and socially, but also different frustrations, personalities.
how often do we live within the realm thinking that what we do/where we are is sooo important. It is, I guess. but it can change. and change quickly.
how does this reflect on our relationships? are these as disposible?