I can't tell you how many times people have asked me why I bother to modify my car. The sad part is, that even with so many opportunities, I have yet to come up with a good answer.
Before I even had a car I was planning my modifications to the car I had planned on getting. I would look through every car magazine, I would order catalogs from every aftermarket company I could find. I would write out long lists of parts, deciding which ones were the best and how each part would work with the others. I tried to figure out how I could get the cool parts that weren't sold in the United States, and how I would make them work on US model car. I didn't even own a car yet, but I was already bench racing it.
As you can probably guess, once I got the car, and some money, it was GAME-ON. It started easily enough; suspension, wheels, exhaust and it just kept going. I never even thought about why I was doing it, it just felt like what I needed to do. I couldn't imagine not modifying a car that I owned.
This has continued with every car I have owned. I have even modified my family's cars for them. Something on their car would break, and I would immediately suggest an aftermarket performance replacement. What, you have a blown shock, let's do a suspension kit, I promise you will love it, and sure enough they did. You need new pads and rotors, well why go with stock when you can upgrade? An order is placed and the car stops better than ever.
I have, either fortunately or unfortunately depending on your point of view, gotten my girlfriend into the same crazed state with her car. She started planning before she even saw the car in person. She has all sorts of parts picked out, now all she needs is the money to make everything happen.
None of this answers the question of why I and others around me like to modify their car. I am not sure there is an answer, why do people climb mountains, ride roller coasters, read books do anything? Because they can and for some reason they get enjoyment out of it. Do I need a reason?