The sad truth of life.

The sad truth is that there are some people who will only be there for you as long as you have something they need. When you no longer serve a purpose to them, they will leave.
The good news is, if you tough it out, you'll eventually weed these people out of your life and be left with some great people you can count on. We rarely lose friends and lovers, we just gradually figure out who our real ones are.