If you comment on a diary, to say hi etc then you will automatically be "watching" it and should receive notification. If you want to stop watching you click "unwatch thread" at the top of you click into it; where you see some trendy little sunglasses this means you are watching it.
Although I have just realised that having replied to you this, for some reason hasn't worked here and I would need to manually click "watch thread" at the top. Yet despite not doing so I still have the sunglasses. Go figure!