Many seeds have a natural defense against being digested. The nutrients we get from them can be very good for us but just like you, they’re trying to survive. So over time, they’ve developed compounds like phytic acid and tannin to make their chances of survival through an animal’s system relatively undigested giving it the best chance of sprouting. Unfortunately, this can make it somewhat uncomfortable for you as the eater.
In addition to their natural defense, seeds and nuts are loaded with fat and fiber, which can lead to diarrhea, bloating, and gas when eaten in excess. If you’re anything like me, these aren’t welcome feelings.