When I first went to OA, I was miserable. I didn't even realize HOW miserable I was! Relationships were strained, my sense of self sucked, and I had absolutely no excitement about the future, much less about my life right at that moment. AllI knew was that I wanted help with eating and losing weight. That was it. That was my entire goal.
Now that I've been abstinent for 7 months (as of today!) from white flour and sugar and have really, intensely addressed my issues with food, I'm the happiest I think I have ever been. I'm really excited about my life, now. I wake up and I'm ridiculously happy. I've even said that I don't know if I'm allowed to be this happy, because everyone else in the world seems miserable. I have fantastic plans for next year, five years from now, ten years from now, when I'm 80...I no longer dread "having" to live through all those days to "eventually" get "somewhere good". RIGHT NOW is fantastic, and I believe tomorrow will be fantastic, too.
I don't know if there's some direct chemical reason that weight would make you grumpy, though I do know that sometimes the endocrine system has problems keeping things level and steady in an overweight body. BUT I do know that the reality of being overweight is that everything is harder--physically and emotionally. Moving, negotiating the world, bending over to pick something up, keeping up with a full schedule, even breathing: everything is harder when you're heavier. That would make me grumpy, too!