Why are the Brotherhood of Steel, the Enclave and the Brotherhood Outcasts hostile to each other? What causes such hostility?

The Brotherhood and Outcasts disagree on what their duty entails, and tempers run high among many of their members. Despite this, both of them despise the Enclave, familiar with the events in the Core Region decades ago and their more recent actions.

I believe in one instance a group of Outcasts run into a group of Enclave deserters who surrender. Their response it, "We're not Lyons' pack of softies. We're the real Brotherhood of Steel. This is how we deal with people like you."