There seems to be an ephasis to feel fantastic all the time, so please allow me to state why that is not a great thing.
To clarify: you should not live in a constant state of depression. To assume that you are broken because you are having an off-day is wrong, though. You need peeks and valleys in your adventure through life.
The way I look at it is that you need a 1 out of 10 day to make the 7 actually matter. If every day is a 7, it becomes a new 1. If the worst thing that has happened to you is you forgot to stop the toaster on time, then the suprising death of a friend is going to hit harder than it should. All of a sudden, that bad day is going to become the worst thing ever.
Again, living every day at a 1 is not great. You should talk to someone (whether it be a friend or a family member) if you cannot seem to shake a funk. However, if you fluctuate your emotions on a regular basis, I am under the impression that you are healthy.
No, I don’t have sources. I am not a professional on this topic, and I make no illusions that I am. I do think that being sad for a day or two is healthy. Getting unreasonably angry from time to time is normal. No one should ever live in a state of bliss all of the time. There have been many dystopian novels that emphasise that idea. Look at Brave New World, in particular, to see how such an existance could effect civilization.