In my case I believe it's because I have waves of enthusiasm. As strange as this might sound, anime has become an integral way of life for me, I don't actively pursue or love a particular series, I just watch it because I have always done. Obviously I enjoy the series I see, but I don't receive the same inclination to catch the next episode. I think it's also because I watch multiple series at the same time. Whilst this enables me to catch everything each fall (this innate desire not to miss anything is an OCD of sorts), it also means I cannot immerse myself into the world of the characters as well. At one moment I'm watching Luke from Tales of Abyss sword fighting and the next moment I'm watching Kannagi moaning about being hungry. Multiply that several times over and you start to lose the connection.
I download almost all of my anime from Winny as MKV, usually removing the files after I've seen the anime, or not, depending on how I rated it. As I've stated in the past, I've also seen so many series over the years that there's very few that provide anything new.
In essence, anime is food to me, I enjoy eating it, but there's only so much variety. I know what I like eating, thus I eat it. It's very rare to come across food that I've never eaten before but end up loving. If I did, that's when I become addicted.