Hi Sasky!
Looks like this thread has been inactive for a couple of months 
And no, you are not allowed to quote me, or use my statements as a refernece, for your study purposes without my written prior approval on this same forum.
Wow that was the official part! My reason to collect what I collect (I do not mean stamps but the countries) are a combination of things:
- emotional (have to choose something that interests you somehow so the heart plays a role here)
- financial: is the area/period concerned fitting my financial situation?
- personal interests: I dig history, foreign cultures, and stamps per se of course
- practical: living in Germany, collecting German stamps offers the greatest market possible of Germany-issued stamps being available, including specialised literature. Actually thanks to, or rather, due to the internet this argument becomes somewhat redundant now I think of it, actually 
I do not know if all stamp collectors make all these decisions consciously. There are different types of collectors after all: some people want to have a nice relaxing passtime without too much ado, and others turn into geeky freaky nerds (like I did I fear) and want to know every single detail of their collection area.
I'm sure you will have found literature by now on the nature of stamp collectors, and more in general, on the psychology of collecting
Cheers,
Kaikai99.
PS I think the reason in the first place I replied here was to know the results of your thesis, but so this may take some years, I guess. Please do report back here in 2011 or so