Who the hell flicked the "summer" switch so early? Wasn't it cold and rainy a week ago. It's 10:00 a.m. and I'm danger of breaking a sweat. If it's going to be this hot I want to be sipping a vodka tonic and laying by the pool and I'm not gawdammit! 
Being a monster, he obviously had an abusive and neglectful childhood (Monsters aren't exactly known for their compasionate parenting).
The only time he peacefully shared with his mother was when they baked and ate cookies. So the only thing that can make him happy is consuming cookies. He can think of nothing else to make him happy.
Unfortunately, years of mass cookie consumption lead to an acute case of diabetes, so he can't even eat cookies anymore, he just chews 'em up and spits 'em out.
And when he's not eating cookies, what is he doing? He walks around looking for "cookie, cookie, cookie."
But he's not looking for a meal, he's looking for his happiness.
Pity the cookie monster.