I started watching it because I saw an increased amount of uploaded images on a certain image-board website featuring the girls from K-On!. It seemed like everyone liked it and it was very popular, so I figured I might as well give it a chance. Also because it was the first anime I've seen with girls playing any sort of musical instrument. (It was also the first time I watched an anime just because of images that were uploaded on a website).
The first episode had me half-hooked. Scanning through the first ep. just to check it out (Start, mid, end), I was thinking "Meh". But after I watched the whole thing I thought it was pretty good. Put simply, as I began to watch more, I became more attached to it. Relating Yui's progress with learning the guitar to my own progress got me even more attached. xD
I keep watching it because it continues to inspire me to play my guitar and allow my appreciation for music to thrive. My old guitar used to lay in a corner and collect dust, until I started watching this. I began practicing again (what was once a month, became once a day), learning new chords and techniques. It even had me buy a new guitar! Also experiencing, their endless funny antics, ups and downs, and idealistically happy lifestyles served as an extra bonus. I mean, who wouldn't mind leading lives like the Light Music Club? Drink tea everyday, play music all the time with your close group of friends, go on vacations and such.
(BTW Filo, 17, In Aus)