I would just like to second what others have said. I often don't feel like getting out for a ride, but I almost always force myself to anyway, for the sake of trying to maintain fitness. For some reason, those days often turn out to be the best, either in terms of performance or simply in terms of clearing my head and bringing some needed calm into my life.
Other days I think I'm going to go out and ride hard and it simply doesn't happen. There are so many variables: quality and quantity of sleep the night before, food intake (quantity and quality), stress levels, amount of allergens in the air, etc.
One thing is certain though. When I ride I really don't think about anything but the rhythm of the bike. I suppose it's something like meditation, but it has a way of re-energizing me that nothing else has.