One theory is that when your body receives foods that are allergic or hurtful, your brain sends out endorphins so you can deal with the assault.
Just like when you break your arm and your body sends endorphins so you can deal with it. Same interaction that exercise addicts get -they exercise so hard that they cause pain and damage, but then they get a super endorphin high so they keep going.

What is interesting is that people have much different endorphin responses, and families with addiction history are the ones who get the biggest high because of their preexisting endorphin levels and how their system sends out floods of endorphins.

People who have low levels of endorphins have many, many receptors. (Makes sense to have a lot of receptors because then you are more likely to get the hit.) But then when you do something that floods with endorphins, you light up like a pinball game!!!