Aaah, the dreaded gquit. To some of us, more used to loyalty, this seems like a sin, specially in some cases where the guild in question that you are leaving has done nothing to make you wish to leave. I had a similar experience before I joined my current guild, except that I was going to lose some friends and gain some more. My guild had almost entirely broken up, to a point that at best there were 3-4 people online, and gquits happened all day, yet I was unwilling to type the command that would end my association with the guild, why....because the guild in itself....a faceless, selfless entity had only provided me with a home, a place to reconvene with my friends, a place where there was always someone who would greet me, despite however hard their day might have been. Infact it became so private that even whispers were no longer necessary, instead we would talk openly in gchat, but I knew that I wanted more (read raiding and more friends).
This is an important decision to make, what is it that one seeks that your current guild cannot provide, more friends to hang out with, raiding, loot ninjaing, griefing, PvP-ing.....there is always a guild to cater to your needs. I wanted a guild I could call home like my current guild, and also a place where there were more than 1-2 people online. I enjoy hanging out with guildies, running instances with them, group PvP-ing with them, raiding with them, these were the things my guild was unable to provide.
Therefore I went out seeking greener pastures for me and my 2 friends. In the end I found a gem of a guild, where I am still at, filled with wonderful people, and even managed to get one of my friends to come. I still feel sad my other friend didn't come, she was unwilling to submit an application for anything that was in a game. I wish she had reconsidered, even though we still stay in touch I miss seeing her name in gchat. Still, I made lots of new friends, frenemies and loot nemesis, but a full screen of friendly banter, a feeling of belonging which came with my new guild is irreplacable. Its a leap some of us have to make, but sometimes we jump into the fire, sometimes into a cool breeze.
Just look before you leap.