I've been in the same job for 10yrs. In many ways it's perfect for me. 10 minute drive from DC school. I get on very well with my colleagues. Company car. Hrs that suit me. Moneys OK.
BUT... it can be very stressful and after 10yrs I find myself getting more and more bored. Sometimes it feels like I'm wishing my life away just to get through the week. Most days I'm just going through the motions.
I've been thinking more and more about quitting and going for something new. What's holding me back is losing the company car (although I live in a city with plenty of public transport, so not essential). I would miss the people I work with too. Also what I have in mind is not a sure thing. I'm 80% sure it will work out but if it does, I will be financially better off and happier (hopefully). If it doesn't, then I quit a good job for nothing.
YABU - keep the security and stay where you are.
YANBU - take a chance at something better.