We have discussed this before although that question is only tangentially related to this one. Based on that question though, you should find that the most active people who answer questions have some of the fewest questions asked. For the most part, the community is split into two broad groups -- those who ask questions and rarely answer (which seems to be where you would place yourself) and those who answer questions and rarely ask (which is where many of the people who will likely respond to this meta post would place themselves).
I can't speak for everybody so I'm sure I will miss individual motivations. But I know for some, they answer questions because they like to teach others. These folks tend to be current or retired academics. Which makes sense, they are making/have made a career out of educating others. And of course, there are those who plan on going into academics and would like to make a career out of teaching others so they could be in that group.
There are those who see answering others' questions as a way to learn more themselves. I know I have learned quite a bit by answering questions I am not an expert per se in but my knowledge of my field has allowed me to bring a particular perspective. And in those cases, I answer something but I also learn from the resulting votes/comments (was I right? did I miss something?) and in some cases has actually led me to ask questions of my own. I also get to see how others reach a conclusion/answer and I almost always learn something from the other answers even if I "knew" it. Turns out, there's usually more than one way to think about something and I wouldn't know how my thought process is different than somebody elses' without having first outlined mine.
I could also say that I fit into a category where I want to encourage others to ask questions and understand how the universe works. It's just a nice thing to do, and maybe that's the goodwill/altruism you mentioned.
There's also the game aspect of it. You ask good questions, you get rep; you post good answers, you get rep. You get badges. You "level up" and get new permissions. It's sometimes fun to play the game for some people. Answers get more rep than questions so that's a good way to advance faster if you're good at it. Plus there's always that "Yay, somebody liked my post!" when you get an upvote.
On the flip side of the coin, like you said about yourself, I spend a lot of time answering my own questions. But that's because I'm in grad school and I need to develop the skills to both develop my own questions and then answer them. Things that I have questions about, I can't openly ask questions about -- they are things I am being paid to research and I could get scooped if I start revealing what I'm doing to the world before it's done. My field is specific enough that what seems to be relatively general/generic questions would turn up in a search engine and it would be easy to piece together things. Of course, by the time I finish working on it enough that it could be public, that's because I've already answered a bunch of my own questions and have new ones that are my next paper! So it's not easy, nor appropriate, for me to be asking questions that directly benefit my work.
So my questions tend to be curiosity about things. And I ask them here and not somewhere else because I have gotten to know the users here through my interactions (either in the chat room, here on meta, or through the Q&A themselves) and I like them. I know that I can ask something and I usually have a pretty good idea who will attempt to answer it based on what the question is about. And I know I can trust the resulting answers or trust the comments that point out my misconceptions or point me to the right answer.