OP... Do you mean it is ALWAYS a factor or CAN BE a factor?
I agree with the second statement.
I agree M&H. We have to be very careful with any certanties. In science/ social science EVERYTHING is open to revision.
I think it's so important to realise that MLC is not ONE thing with a single trajectory, but a set of behaviours with some similarities. In research, we have to be so careful about NOT fitting the facts (as we see them) to existing theories, but to be open to other interpretations. So, although it is comforting to apply a theory such as "it's hormones", and it CAN sometimes be relavant, it is not always.
Our brains and bodies work with chemicals/ hormones etc of all types; any changes in their balance can disrupt they way we function. (For example, food intolerances can lead to depression, irritability, etc.) Humans are also social animals, and we build our perceptions of ourselves and the world through social interactions (although their is a pre-social feeling "self" in the limbic brain). These perceptions also have an effect on the physical body, neurologically, and chemically. So there is an interaction between the sociocultural world and bodily reactions of all types.
Sometimes intervention is needed; psychiatrists know that they need to intervene with antidepressants before the brain learns to create depressed behaviours. HRT is sometimes important to avoid secondary effects of a sudden change in hormone levels. But drugs, as M&H has said, are not always the answer. Even when they are important, they are not the only factor for psychological problems... as we all know, because our attitudes count too!