When I first had the thought, I developed a basic mental chart of emotions. Anger/hate, sadness, fear, love, happiness. These are things I consider to be the emotions which we can basically call other emotions synonyms of. There are other things that people consider emotions, but to me, all those emotions require one of these emotions.
For instance, some might call 'pain' an emotion. But pain is a stimulus FOR emotion in my mind. Same for shock/surprise. It is a trigger for an emotion. The trigger for the emotion can produce different words ('horrified' for instance is an emotion from the stimulus of shock, to something that you react negatively to), but is not itself actually an emotion.
Maybe if I searched long enough, I would find some other emotion that I think is definitely separate from those five (you could make an argument for a lack of emotion to itself be an emotion, e.g. apathy, boredom, but I don't really count that), but I think those five emotions tie to everything.
But even more...I think that they have a distinct relationship, in that there is only one emotion that cannot have an emotion underneath it. I figure that all emotions are layered, and even the base five are no exception: they can have an emotion underneath them driving them (save the exception). This underlying emotion may itself have an underlying emotion, and so forth, until leading to a root cause, a root emotion, that serves as the absolute deepest, fundamental level of that top emotion.
And what I found was, even though there are five base emotions as I see it...only three can be on the bottom: Love, Fear, and Sadness.
And Love is the exception, the only emotion that cannot be higher on the chain, because by its very nature, it cannot exist elsewhere. After all, if love had an emotion driving it...it wouldn't actually be love, no matter what emotion that may be. (Not even positive emotions, like happiness.)
As for the other two...Fear is a primal emotion, but does not necessarily have to be at the bottom like Love can. It can be at the top, in the middle, or at the bottom, anywhere, really, because fear is literally hard-wired into our brains as the fight-or-flight response. Love can cause fear (but not vice-versa), sadness can cause fear, hate can cause fear, and rare as it may be, so can happiness. (Though, in the case of hate/happiness, they eventually tie back to love and/or sadness anyway. See below for more.)
I absolutely refuse to believe hate can exist as the lowest level. Something needs to create that hate (usually fear or sadness, but love is possible), and from that hate, some other emotion can emerge, such as fear. This, in some cases, can of course cause a cycle: one creates another, such as a fear-hate loop.
But I do believe these emotions always have this relationship. As for why happiness isn't at the lowest level...my firm belief there is that happiness stems from love. It can get tied to sadness, it can get tied to fear, but happiness cannot, fundamentally CANNOT, exist without love first. Now, mind you. This love may not necessarily be the traditional definition. There's plenty of different loves. Love of oneself, love of one's life, love of others, love of an object, love of a concept, love of something. To put it a different way, in a sense, love IS happiness.
I consider them separate emotions, but I think that by certain definitions of love, happiness will always stem from it. Which is why, in spite of being one of the prime emotions, it cannot be the base emotion.
We may not know what it is causing these emotions, such as happiness. We may not know what causes, say, love to sprout, and the emotions from that love seeping out. Emotions are complex and only beginning to be understood. But I feel like this is at the heart of the complexity of emotions, this system where something causes one of three emotions, which mixes with the something and with other emotions to create that higher level of emotion.
This is, of course, not an open book, far from an exact science. But I still think they work on this system all the same.
It's a shame nobody really comments on my blog (oh well, not much which can be done about that), because this is something I'd love to talk about more if I had a better focal point which someone asking me questions about what I've said would give me.
As of right now, not quite sure where I was really going with this, but I thought it an interesting idea all the same.
Basically...there's a difference between stimuli and emotions; emotions result from stimuli but stimuli are not themselves emotions, in spite of having multiple emotions named from them. All the emotions we know, are mixtures of stimuli and other emotions. These mixtures exist on levels. When undiluted, there are five base emotions, but when traced back to their base level, only three of them can be the absolute bottom layer.
Sadness, much as it may suck, is one of them. Fear, as a primal emotion, is another. Both can exist on other levels as well. But love, the third deepest-level emotion, cannot, and is where the majority of our drive comes from. Hope, happiness, conviction, and many more, all trace back to that love.