Im sure this question has been asked tons of times before but i am curious. Why do you mine?

I can see a few reasons:

1.) (and hopefully the most common) you genuinely want to help BTC/LTC/etc become a legitimate currency in everyday life.
this seems like it should be the most popular answer. but eventually you have to do something with all that mined coin. and with the difficulty building all the time, it seems like a common part of mining is to immediately use your mining proceeds to get more mining power. that way you can actually make back the initial investment and pursue one of the other routes to profitability while still helping bitcoin prosper.

2.) youre in it for the money
you see the btc-usd calculators and just see dollar signs with no real effort other than the initial investment and the occasional power bill. not to mention the actual act of turning that btc into usd which seems like it can be trying at times with all the steps that have to be taken. especially if you are just selling on exchanges rather than to people via craigslist or localbitcoins or similar

3.) youre in it for the money : hardware version
you mine to make btc to mine coins to buy hardware to sell. This ties in to #2 but with the specific focus of being all about selling hardware. This seems to be the most profitable but that is slowly coming down. anywhere that you can get miners for BTC (which to me seems like a more trustworthy business than those miners that take USD, but thats a different topic) you can hoard them and resell on ebay or craigslist or wherever. those 2btc erupter usbs were selling like hotcakes (why do hotcakes sell so well?) at 400 bucks a pop initially! now they have cooled to closer to 1btc (after the price of them dropped to 1btc) rate equivalent and often you can get them for less than 1btc equivalent in USD. this is becoming a problem now because you are often hit with fees so an ebay auction is around 13% when its all said and done. at those rates, you would just be better to use localbitcoins.com to sell your coins but then its way more hassle. 

what other categories have i missed? why do you do it?

personally, i am mining for coin just to mine more (it seems to be addicting to always try to get better hash rates :/ ) I have not done anything more with my coin other than reinvest but the time is coming soon i guess to figure something out.
ive got an erupter blade, an erupter usb army thats slowly growing, and an fpga single for nostalgia. my gpus are only hot when im playing farcry blood dragon or something.