Your characters come to life, you can see and hear them in your head, and you know your story is “alive”, the way good stories should be.
When you’re editing your work being able to feel your prose “tightening up”, becoming sharper and so your work improves.
Writing can be marvellous therapy in terms of getting away from whatever horrors life drags you through.
Meeting other writers!
Never having as much time as you’d like to get on with said writing.
When it’s a struggle to get the words out.
When your work keeps getting rejected.
Not getting replies from agents/publishers despite doing everything right when submitting work.
Just knowing the only one who really cares about your work is you!