There are a couple of different things that really suck about losing good friends. One of the first can be how you lost them. Maybe someone moved. Maybe someone messed up. Someone got angry for no good reason and just messed up. Maybe you just drifted apart. People get busy with life, or find other people they're more interested in. Maybe someone got tired with life.
But in any of those cases, it still hurts.
For one thing, you miss all the creative stuff you'd done together. You miss the plans that you made, the doodles you'd done together. You miss the way you used to get excited over things with them. A book. A show. A movie. A webcomic.