|MadSci Network: Chemistry|
Abigail, thanks for the question. The answer to your question lies in your statement that both writing paper and paper towels are "just paper." When paper is made, there are a number of things that can be done to alter the natural properties of the wood fibers to help the paper perform better in its end use. For example, you wouldn't want your milk carton to fall apart like toilet paper when it gets wet. So, the paper in the milk carton contains additives that help it hold up when it gets wet.
Wood fibers are naturally hydrophilic, which means they like water. Because of this, paper towels are effective in absorbing water when you spill it. Writing paper is treated with chemicals called sizing that give the fibers a hydrophobic nature. Hydrophobic substances don't like water, so they keep the water away from the fibers. The writing paper is treated to help the paper survive in the elements and also to keep the ink at the surface when you write so it does not look faded.
I hope this clears this up for you. Just remember, paper is rarely just paper.
I answered another question about paper towels that might be of interest to you at http://www.madsci.org/posts/archives/sep2000/968769784.Ot.r.html
Try the links in the MadSci Library for more information on Chemistry.