What makes writing emotional?

Alright, so to infuse personality into a paper, there are lots of things you can try.

This might seem silly, but it works. Read something unusual. If you're having problems because your prose is too dry, read something so soapy you could scrub the dishes with it. Do you normally read scientific articles? Read a graphic novel. Do you normally read classics? Read a thriller.

You can also try to read things by authors that have high technical skill, but are also known for something else. Charles Dickens, Mark Twain, and J.R.R. Tolkien come to mind, along with many others.

Also, vary your sentence length significantly within each paper. It helps. Fragments might be grammatically inadvisable, but for slamming home a point, they can be invaluable. If you use it too much, it just becomes a cheap trick, but at the proper place, at the proper moment, it's more of a flourish. Use sentence length to compliment your intention. Do you want it to flow smoothly, and relax the reader? Or do you want it to jolt them, and jerk them, and keep them alert? The same goes for word length. Using multisyllabic words is no bad thing, but sometimes the smaller words are better. A little asceticism can go a long way. Using words with stop consonants can also have the same effect, but it's less constant. Using lots of b's and p's doesn't really work, but k's, g's, and t's can make a sentence "harder," if you will. D's are on the borderline.

In the previous paragraph, I wrote the sentence "It helps." towards the beginning. Replace it with "It will aid you immensely in manipulating the reader's emotions" or something and see if you think it changes the tone. Or compare "You're being stupid." to "Idiot!" Think of all the synonyms for "idiot" you know of, think of how/why you'd use them, and then try to analyze the differences between them. Unless you don't feel emotion when you insult people, of course. :)

As another example, allow me to shamelessly steal the example from Claudiu's answer:

"I think the king's policies were ridiculous. How could he possibly think of continuing to raise taxes when his peasants were starving?"

Instead of trying to make it sound smarter, try to make it sound harsher. You are, after all, making a rather pejorative judgement:

"The king's policies were idiotic. In his desire for power he continued to increase taxes, without regard or respect for his people's inability to feed themselves, let alone further fill his coffers."

I replaced "ridiculous" with "idiotic" because it has a faster, staccato rhythm, partially due to the facts that it has fewer consonants overall and what few it has are all stops.

Another trick I used there was stating the opinion as a fact. I don't think the policies were ridiculous, they were quite inarguably idiotic!

The other answers have some excellent advice. I hope this helps as well. This is already too long, but if I think of anything else incredibly useful, I'll add it.

EDIT: I should have said this first. In your writing there are bound to be lots and lots and lots of little things that make your writing yours. Look for those things. When you find them, build on them before you try to take any suggestions you find here. No matter how good someone else's advice may be, it's still someone else's, and adopting it can make your writing just seem artificial. Try to develop the personality you already have, not the one your professors think you should have. Even if it makes your writing less than "perfect," if it's honestly written, they'll appreciate it. And if they don't, their opinions aren't worth taking seriously outside of how it affects your grade.