How does music express emotion?

This is true shapeless. I tried to find a reason why your wife would not find Beethoven's 'eroica' heroic. However, far be it from me to judge a musical composition by its historical context. Art is timeless by definition.

Is this counterintuitive to what I just posted? No, not necessarily. In order to appreciate a certain piece of art, it's not necessary at all to know its historical context. You can just look at the 'ding an sich', so to speak, and not relate to its history at all. Certainly, this is how I enjoy most of the classical compositions I listen to (and those are few... I am a victim of the pop - era ).
However, in order to fully appreciate a work of art, it's necessary indeed to understand the creator's motivation and inspiration, and that, unavoivably, leads to looking at the original historical context.

Does this clarify my two cents?

Within the confines of a given style, and with sufficient understanding of how an instrument is played, it's possible to process it logically and objectively to some reasonable degree.

Why does Music Evoke Emotions?

The Theory of Musical Equilibration states that in contrast to previous hypotheses, music does not directly describe emotions: instead, it evokes processes of will which the listener identifies with.

A major chord is something we generally identify with the message, “I want to!” The experience of listening to a minor chord can be compared to the message conveyed when someone says, "No more." If someone were to say the words "no more" slowly and quietly, they would create the impression of being sad, whereas if they were to scream it quickly and loudly, they would be come across as furious. This distinction also applies for the emotional character of a minor chord: if a minor harmony is repeated faster and at greater volume, its sad nature appears to have suddenly turned into fury.

The Theory of Musical Equilibration applies this principle as it constructs a system which outlines and explains the emotional nature of musical harmonies. For more information you can google Theory of Musical Equilibration.

Bernd Willimek

I was reading the comments and replies to this question. I noticed that everyone seems to be answering the question by the way of music theory and history. I have a different twist to answering this question using modern day music. Slower tempo generally makes me feel either sad, relaxed, romantic, or sensual. Faster tempo generally makes me feel excited and pumped up. Each specific musical instrument can also cause a emotional response from the actual sound of a specific instrument. The sound of a piano may have a different emotional response from electric bass. The sound of a lead rock guitar may have a different response from a violin. The sound of a saxophone may have a different emotional response form a tuba. You get the idea.