Because managing fleets and watching them grow is fun and if your a good fleet leader, you get free access to tons of items for nothing that people will just put in there for anyone to use! Plus being in a fleet makes the game more interesting and makes mission grinding funnier and more exciting. Yea a ton more then one word, but I had to explain why I said what I said. :p
Currently, a DOFF addiction (which had already hit full swing before this weekend).
In general, because I like flying a big Star Trek ship around to places whose names I recognize from Star Trek. That's as simply as I can explain it. Plus I've paid for a lifetime membership so I may as well play.