This all comes down to if you want to play it safe (stability motivates you), or if you want to roll the dice and gamble (change motivates you).
I speak from experience. I made a risky choice in 2000 and joined a startup, quitting a secure job at IBM that I would (in all likelihood) still have today. The job I went to paid better, was a lot of fun, exciting, challenging, and in the end a failure. My career has never fully recovered, and I am certain that had I stayed at IBM I would be finincially way further ahead than I am now. By all reasonalble criteria, I should regret my decision.
Yet I *had* to do it: I crave re-invention and change. I wouldn't be happy stuck in the douldrums of a stagnant work environment. I work for myself now, but I have no problems envisioning myself going back to being a cog in a big machine again. I'm open to, and embrace, the possibilities.
But as for you, you have to make that decision for yourself. The operative word about your job is not "fun", it's "happiness". You're in a fortunate position of being satisfied with your career, so you need to decide if you will regret not taking the opportunity to do more (and risk that you will fail). Good luck.