Several things would count in my case:
- Confidence ("well you're qualified now, you make all the decisions")
- Getting down to FTO where the aircraft was booked in not-so-great weather; being told to take it up or cough up for a cancellation fee; and finding from the tech log that all the other recently-qualified PPLs have done that day is 1 circuit and brought it back again.
- Actually wanting to get to ME/IR but can't afford to spend that sort of money
- Getting bored doing the same old routes that I did during training
- Watching the regulation getting tighter and wondering whether it's worth carrying on
- Getting shouted at/humiliated by ATC for asking for clarification of an instruction (yes, it really happened and I actually had an instructor beside me at the time who didn't understand what we'd been told either).
At the moment it's lack of time that's preventing me getting back in the LH seat, but I could quite easily also count the reasons above if I'm honest. By next spring I will hopefuly change from passenger to PIC again!