I was recently in a conversation about romance, where it was suggested maybe "it's all just hormones." I reflexively responded with a pat answer which I can't bear to reproduce here, because in retrospect it strikes me as scientifically and philosophically careless. This seems a sign that the formal philosophy part of my brain is disconnected from the ordinary conversation part of my brain, a fact which could be the basis for many significant philosophical ruminations. However, that's not what I want to talk about today, rather, my concern is to get a more accurate view of what we should think of suggestions that something is "just hormones."
Let's start with trying to get the scientific issues straight. It's been suggested that the key to multicellularity is communication between cells, and in animals (including humans) the two main systems for doing this are the nervous system and the endocrine system. In the nervous system, signals are transmitted within cells by ions, and between cells by neurotransmitters (mostly--ions are sometimes used for cell-cell communication too). The endocrine system is everything involving hormones. This doesn't entirely cover all of our known mechanisms for cell-cell communication, since there is also shorter-range hormone-like things (paracrines), movement of chemicals through gaps in the cell wall, and direct interaction between two membrane protiens. But everything we know about what underlies how our bodies work is underlied by chemicals and ions.
Perhaps there is some room to think there are other things going on, I'll get to that in a moment, but let me first emphasize that even if there is more to our actions, we can be sure we could not act in any way, even for a few seconds, without these ions and chemicals. Even if our minds were entirely cared for by a Cartesian Mind, with no need whatever for nerves to be involved, we would still need the chemicals and ions of nerve signals to move our muscles--specifically so that calcium ions could enter our muscles, causing the protein fibers within them to contract. Hormones are more than just sex hormones, they also include things fundamental to our metabolism, which, when not working properly, are often responsible for serious diseases. Remove them entirely and we would quickly die--perhaps, in a delirious state markedly different than our normal mental lives.
So, what might there be to our minds beyond these material entities? In contemporary philosophy, the most popular candidate is consciousness, what it is like to have certain experiences. I won't take the time to comment on the reasons for this position, beyond the fact that it seems to me at least respectable: we know of our own consciousness in a very intimate way, and at first glance it seems a very different thing than matter. A great many philosophers who promote this view (Chalmers, Kim, Jackson), though, are epiphenomenalists. They think consciousness is caused by physical events without having any physical effects. If that's true, then everything we do is because of chemicals and such, even if there's more to the result. In a romantic relationship, for example, there would be what it is like to be infatuated, what it is like to kiss, what it is like to have an orgasm, which cannot be reduced to physical events.
The third alternative to physicalism and epiphenomenalism is interactionism, on which our non-physical mental states can have physical effects. However, it is not clear what the consequences of such a view would be. Would we have free will? Consciousness could be governed by deterministic laws just as much as the physical realm. And if the effects of the mind were not determined, what would they then be? Random? Arbitrary? Are random or arbitrary actions anything to celebrate?
The above discussion is about events, but what about personality, broadly defined as everything that is lasting in a person's mind? Here, I cannot see that it is just obvious that there is something radically outside anything what science currently has a handle on. Arguments to that effect seem to me to mostly stem from a failure of imagination, a failure to consider Hume's suggestion that human personal identity is no different than the identity of inanimate objects, that we have nothing more significant than persistence of a ship having it's parts replaced. There is also much evidence that physical changes in the brain affect personality: I have first-hand experience with what drugs (including alcohol) can do to a person, and what brain damage did to my grandmother. Today in my neuroscience class, the professor talked about dealing with his son's schizophrenia, about the genetic component, about how wonderful some medications have been in treating it. One might reason that if there is more to our moment-to-moment life than the physical, there might be more to our persistence over time, but this remains in the realm of bare possibility.
After all of this, what exactly is it that bothers us about something being a matter of hormones? I think it is the arbitrariness of it. They might have been otherwise, but it seems odd that a change in chemical would change what we ought to do, so actions based on them are not particularly likely to be the right one. What more do we want, though? I am not sure that most people can really say. Yet we must act anyway. Thus, as is the case with the problems raised in my post On not knowing thyself, we act without perfect knowledge of our situations. There are a couple variant strategies here: if you like being confident, you can be a Moorian, and declare that you know with absolute certainty the correct answers to many important problems, even if you do not know the correct analysis of many of the relevant concepts. If you are more humble, and less worried about consistency, you can be a Humean, and make carelessness and inattention your remedy to your problems. I myself am very divided between the two.